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