I would recommend you review mathematically what is going on in telephony:
-channel
-receive/transmit
-protocol to establish a call: calling party and receiving party, cooperation between them is a must.
-call numbers and telephone directories : either you cooperate and your number is public/published, or you don't and your number is not public
-calling party line identifier: you cooperate by making the number you are calling from visible to the called party. You may not cooperate and not show your number. I usually reject such incoming calls because I find it rule to mask who is calling.
You find technical details in any Communication Textbook, or at ITU-T which has specified voice telephony.
Based on the above, you may consider other use cases like collaborative online document edition, online meeting tools with document sharing. All these with two or more participants collaborating.
I also recommend you look at CBPP commons based peer production, which is a generalisation of Wikipedia : volunteers, in interest communities, cooperate to publish documents or other digital objects.
This is about cooperation and accepting common rules for efficiency, accuracy, fairness and balance, augmenting the common good, progressing society as a whole