We are looking to create a bitcoin hardware wallet for mobile devices. This would include a full-node as to not rely on a third-party service. However, we also want secure text-messaging and hopefully VoIP built as a feature.

I use the comparison of a SIM card for comparison purposes only - obviously it would need to be quite different in technical details.

One of the obvious considerations is that to have SMS (160 character) based communications, new features and perhaps OP_CODES would need to be enabled on the blockchain. This could potentially require a hard fork of the software.