Implementing a phone system in game would be interesting. In every general store you would be able to buy various phones: from cheap (low security, not able to do bank transactions). Every player would get a randomly generated phone number.
With a phone you would be able to put bounties on players (not needed to go to payphones) as well as transactions between players, and maybe more depends on what you all suggest to make this suggestion even better (if you like it of course).
While robbing players, except cash, you could steal his phone and if you crack the players phone, you could steal (transfer) cash from the players phone. To protect yourself you would need to block your phone transactions in the bank in order to protect your bank account. If you don't do that then you would receive a message from the bank "Wiretransfer confirmed" which should be limited with I don't know how much. The ability to do this type of robbing depends on your phone, the more expensive, the less possibilty to get your money stolen.
Maybe there can be more with a phone, but this is what came on my mind.
