The payphones offer great RP opportunities by offering a way to call people anonymously for only $20.
Problem is, most people prefer to not carry around cash on them out of fear of losing it, and payphones take cash only, meaning often times people just don't use them.
It would be cool to have the ability to buy prepaid payphone cards in supermarkets, that are stored in your inventory like crates or briefcases, which can be consumed by the payphones to activate 1 call.
I also think this will make it more accessible for people to take advantage of payphones.