1. marital disloyalty; adultery.
2. unfaithfulness; disloyalty.
3. a breach of trust or a disloyal act; transgression.
5. having an affair
In today's world of cell phones, texting and emails there is no reason to be on a payphone other than to cover the tracks of your affair. When you suspect someone of cheating on a spouse you say "they are on the payphone".