Just a little test project for a client I did the other day using B4J and B4A. I may post the code later if I ever get the time.
The basic idea was to interface an old rotary phone with a modern cellphone. So, I used a Raspberry Pi 2 to do the physical interfacing with the contacts/switches on the rotary phone. I used B4J on the Raspberry Pi to read the GPIO pins. I used MQTT on both the B4J and B4A code to 'talk' with each other. The Android Phone is using B4A code to just listen for a phone number and dial it once enough digits are entered. Simple.
There are lots of other things this interface does that's not shown in the video.... such as dialing 411 ("Information" here in the USA) causing the Google search app to open .. subsequent dialing 'types' letters into the app to look up information from the rotary phone. Also, dialing "00000" pops up the Google Maps app. And so on. Anything can be done. Including, having multitudes of the Android (and IPhone) apps running at the same time and calling the same number simultaneously ... from an old rotary phone.
