[GianFabien]

Why not just pull an old Android phone out of the junk drawer, pop in a PAYG SIM card, connect via USB cable and you're good to go. There are several apps that will implement the connection if you don't want to hack it yourself.

[j45]

I was wondering about this too. It could be a an android using something like airdroid and still provide people with the opt out messages sms always should have.

Putting a keyword at the front of outgoing and incoming messages might be able to help as well if a users phone number was tied to more than one service.