I think you should add another use case; situation in which some spammer tries to sell some product by a phone. You could incentivise users to buy this coin by making the reward proportional to the coins they have. Users could not simply end a conversation after few seconds; there should be some minimum of time after which the reward should be proportional with a time. After that time the user could end the conversation by simply saying "no, thank you". However - if the user agrees to the offer - a seller doesn't have to pay anything.