Documentation on how to use Signal with Dual-Sim Phones

Using Signal with Dual-Sim Phones.md

Using Signal with Dual Sim or Multi Sim Phones

Signal (previously TextSecure) on Android can be used on dual-sim or multi-sim smartphones, albeit with some limitations regarding use of the additional sim cards.

Multi Sim Usage

Available Functionality

✓ Signal can be registered with one phone number

✓ Signal can be used to send and receive encrypted messages (addressed to and from the phone number that was used during registration)

✓ Signal can be used to receive regular SMS (using any one of multiple sim cards)

✓ Signal can be used to send regular SMS (using any one of multiple sim cards; long-pressing the send button allows to choose the sim card to be used)

Limitations

⚠︎ Registration: Additional phone numbers cannot be registered with the Signal app. Instead, the Signal app may only be registered with one phone number at any time. Specifically, this is the phone number that has been used in Signal's registration process.

⚠︎ Sending and Receiving Encrypted Messages: Signal can send and receive encrypted messages only from one phone number (i.e., from the phone number that has been registered with the Signal app).

Usage Scenarios

Fresh Installation of Signal on a Phone with Multiple Sim Cards

The following steps describe how to install and use Signal on a phone with two or more sim cards:

Installation: Simply install Signal as you would on a single sim phone by downloading it from the Google Play app store.

Registration: Register Signal with one of your phone numbers, as you would do on a single sim phone.

Sending and Receiving Encrypted Messages: You can then use Signal to send and receive encrypted messages, as you would do on a single sim phone. This means all encrypted messages are addressed from and to the one phone number that you chose to register with the Signal app. As with single sim usage, sending and receiving these messages requires an internet connection. It does not matter which sim card, wireless network etc is providing the connection.

Using Signal for SMS messages: You can optionally configure Signal as default messenging app. The corresponding configuration option can be found in the settings of the Signal app under "SMS and MMS". If you enable the SMS and MMS option, Signal will be used by Android as default app to handle incoming SMS and MMS. Signal will receive SMS and MMS messages from all sim cards and will display which sim card or phone number the message was addressed to. When using Signal to send regular SMS, long-pressing the send button will allow you to choose the sim card to be used for sending.

Adding a Second Sim Card to an Existing Installation

The following steps describe how to add a second sim card to a phone with an existing installation of Signal.

Assumptions and Preconditions: The assumptions for the following steps are a dual sim or multi sim phone with only one sim card inserted. And that Signal is installed and registered on this phone.

Inserting a Second Sim Card: Inserting additional sim cards into the phone does not break any previously available functionality. This means that Signal remains registered as before and that it can still be used to send and receive encrypted messages, SMS and MMS on the default sim card. Nonetheless, usage of the additional sim is subject to some limitations.

Sending and Receiving Encrypted Messages: You can then use Signal to send and receive encrypted messages, but only from one phone number that the Signal app is registered with. As with single sim usage, sending and receiving these messages requires an internet connection. It does not matter which sim card, wireless network etc is providing the connection.

Using Signal for SMS messages: You can optionally configure Signal as default messenging app. The corresponding configuration option can be found in the settings of the Signal app under "SMS and MMS". If you enable the SMS and MMS option, Signal will be used by Android as default app to handle incoming SMS and MMS. Signal will receive SMS and MMS messages from all sim cards and will display which sim card or phone number the message was addressed to. When using Signal to send regular SMS, long-pressing the send button will allow you to choose the sim card to be used for sending.

Three or More Sim Cards

Third and fourth sim cards will work, albeit with the same limitations that also concern the use of a second sim card.

Multi Sim Phones with just one Sim Card

Users of dual or multi sim phones can use Signal with only 1 sim card inserted. In this case, Signal works the very same way as if using a single sim phone.

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Can confirm the described behaviour as well!

Many thanks for the information on using Signal on multiple SIM devices. With respect to having multiple SIMs, you mentioned "You can then use Signal to send and receive encrypted messages, but only from one phone number that the Signal app is registered with. "

Does this require the user to select the SIM for that phone number as the active SIM before Signal will work?

Or will Signal simply work as long as the SIM containing that phone number is present (not the currenly active SIM) in the machine? I notice Signal works even when the phone is in Flight Mode using WiFi.

Thanks.

Hi!

Regarding your first question, "Does this require the user to select the SIM for that phone number as the active SIM before Signal will work?": The limitation that I described in the gist, "...but only from one phone number that the Signal app is registered with", means that encrypted messages can only be addressed from and to the one phone number that was chosen during registration. Hence the selection regarding which sim or phone number will be active (i.e., will be used by Signal) happens during registration.

Regarding your second question, "Or will Signal simply work as long as the SIM containing that phone number is present (not the currenly active SIM) in the machine? I notice Signal works even when the phone is in Flight Mode using WiFi": Sending and receiving encrypted messages requires an internet connection (this is no different from using Signal on a single sim phone) - it does not matter which sim card, wireless network etc is providing the internet connection.

I sought to also clarify this in the gist.

Hi

I have a fairphone fp1 and recently added a second sim card. With the built-in messaging app I can send SMS messages with either of the sim cards, however Signal will only send from the card that's in the first sim card slot. When I long-push the send button nothing happens so I cannot choose the other sim card. Any suggestions?

I have the same problem as @ThorNM

I can send SMS messages with my second sim in the second slot, but I have to long press. I have disabled my first Sim in the Android settings, but it stays available and the default in Signal. This is very annoying.

I had the same problem. I solved it by setting my default app as the SMS app, sending a message to the person I wanted to message with the 2nd SIM card, then changed back to Signal. When I received my reply, Signal had switched to using the right SIM card.

I had Signal before, on my old smartphone with CM12. Now I got a new phone: OnePlus 5, Android 7.1.1/OxygenOS 4.5.10

On the new phone, I got Dual-SIM. The first SIM (SIM1) is a new business SIM which has not been registered to Signal before. SIM1 ist the only SIM that got mobile data connection. SIM2 is my old SIM which is registered with my account/phone number. SIM2 is only used for calls and SMS.

I started a newly installed Signal app on my new phone, registered to my (old) phone number of SIM2. The registration process with a confirmation SMS went smoothly. Then, I was able to send encrypted Signal messages via SIM1 with the account/number from SIM2. I even disabled SIM2 for testing purposes and were able to send encrypted Signal messages via SIM1.

Then I also disabled WiFi (in addition to the disabled SIM2). I could also send encrypted messages via Signal.

Side-mark: Signal is offering both SIMs for insecure (clear-text) SMS when long-pressing the send-button.

So: it seems to be the case that the data connection does not rely on the SIM that is registered with Signal.

It would be really nice to be able to set a default SIM to use for sending old-fashioned SMS messages.

There is a setting in the Android settings to say which SIM should be used, but Signal seems to ignore it. It's OK if you're replying to an incoming SMS, in which case Signal will use the same SIM as it came on, but for a fresh conversation it seems to just use the first one without asking.

I have two SIMs, one one of which the SMS messages are free, whilst on the other they cost 6p each. Signal inclines to using the latter, despite the system settings being set to use the former.

I tried to use Signal. I have a dual SIM phone. My dual SIM phone is so called "dual sim, dual **standby" device. I.e. the 2nd SIM, while not actively used, is always "standby" and able to receive any phone calls or SMS send to the 2nd number.

I only want to use Signal as my 2nd number.

And I registered my Signal with my 2nd number. The installation went smoothly. Again, due to the fact that my phone can receive confirmation SMS easily.

BUT after a while, when I open up Signal app, I got this "Device no longer registered" message across the top. I have to re-register with my 2nd phone number again. This process has to repeat itself every hour or so.

I suspect that Signal in some way is constantly checking rather the phone number is the primary one. If it is not (in my case, the 2nd number is still live, able to receive sms and calls but nothing more than that), Signal seems to de-register itself.

Thanks for a nice howto! I agree with @jhwinters about the need to respect the Android setting regarding default SIm for SMS.

Well, that missing functionality of having both, business and private Messages being encrypted, is a real drawback.
Another drawback i noticed is, that answering unsecure messages via KDE connect will bump them into the default messagestore, despite the fact I deactivated the default messaging app completely. Seeing these my own answers requires to reenable the default app. But maybe that's caused by installation-order or something like that, I had KDE connect installted before signal ...

Anyone have any idea how to switch the default SIM for a contact?

I HAD two SIM's one phone. My contact used to contact me via SIM2. I ultimately got rid of SIM1, moved SIM2 to SIM1, now - I cannot send a text message to the contact. I tried re-installing the app, etc. How can I change the SIM associated with a contact - or delete a contact, for that matter.

Signal does not seem to be compatible with the Samsung's dual messenger feature. Pity because that would make possible to have two instances of signal, one per SIM.

Is it also planned on displaying the SIM's name (and maybe icon too) as configured in android's setting? That would be some explicit than the "SIM 1" and "SIM 2" labels. I can't ever remember which SIM is what.

Thanks.

Signal does not seem to be compatible with the Samsung's dual messenger feature. Pity because that would make possible to have two instances of signal, one per SIM.

Is it also planned on displaying the SIM's name (and maybe icon too) as configured in android's setting? That would be some explicit than the "SIM 1" and "SIM 2" labels. I can't ever remember which SIM is what.

Thanks.

If you have a Samsung device, you can use it via Secure Folder. Essentially, it's a phone within a phone, and you can set notifications to work when Secure Folder is locked. Works great for me.
You will need a Samsung account (ugh, I know), and two Google accounts, one for Play Store outside Secure Folder, and another for Play Store inside Secure Folder.

https://www.samsungknox.com/en/solutions/personal-apps/secure-folder