/phone
This command is the entrypoint for the carrier user. It allows for message sending, calling, subscribing and unsubscribing from carriers, etc.
Without any subcommands, it will return the current signal strength 
Also, if you're in a call, some info about it will be shown (duration and participants) 
/phone subscribe carrier
This command can be used to subscribe to a carrier. It will transfer the cost of subscription to the carrier owner upon running, and add the player as a subscriber.
/phone unsubscribe
It will unsubscribe you from your current carrier.
/phone message player message
Allows you to send a message to another player. If you, or the person you're sending to doesn't have signal, it will not send.
/phone call player
If you're not in a call, it will call a player. If you're already in a call, you will add that player to it. Others in the call will be notified that you invited them to the call.
/phone answer
Answers incoming call.
/phone end
Hangs up, leaves from current call.
/phone info
Show info about your phone, which tower you're connected to, whether you're roaming, your carrier's pricing, which is the best band you can connect to, and the speed of sending messages.