Since I'm not familiar with this particular short story, it's hard to tell who the main characters are. However, it's possible that there could be a protagonist who is expecting an important phone call, and the person on the other end of the line who is making the call could also be a key character. Maybe there are other characters in the background whose actions are related to the phone call, like a family member in the same room as the receiver who overhears part of the call.
I don't have enough information to say who the main characters are in 'the phone call short story' as I haven't read it. But usually in a story about a phone call, there could be a caller and a receiver as the main characters.