If you disable mobile access on Android, you can still use an unprepared offline network. If you have mobile networks enabled and connect to a not prepared offline network, the smartphone will fall back to mobile wifi. Sometimes, there is a message, that gives you some chance to use it, but the Samsung phone I had for test purposes denied to work with LibraryBox (which is not prepared yet).
iOS5 or iOS6 introduced a call-home function, that checks for the internet. If it does not receive a specific answer (see below), it opens up a lightweight browser, where you usually can login in hotels (the captive portal). If you don't solve the connection issue with logging in, I think < iOS9 does not fall back to mobile network. I read, that iOS9 will partialle use mobile internet, when the wifi connection is bad. As vague this statement is, so low my knowledge is. Fact is, my prepared PirateBox does not bring my iOS9 iPhone back into mobile n