Two frustrating things with the phone system atm: no caller ID being shown when someone is calling as well as the lack of possbility of being able to talk off the phone whilst in a call (used /p <text> before the update but now you talk without the command). Also, the /p command seems pretty useless as of now whilst in a call, it doesn't even tell you to talk without a command and so it was very confusing when the update first rolled out.
One change that I suggest adding is the functionality of a speaker - being able to switch it between on/off so, other players can see in chat what the player on the other side of the call is saying. Another suggestion would be a clearer display of the texts/SMS, there's a decent amount of unecessary clutter which affects clarity. These changes are more quality of life than anything, so they're not necessary-per-say but it would improve the enjoyment of using the system.