I have a thought, but it may not make any sense. I'm going to give it a shot anyway, so bear with me because it's hard for me to put my thoughts in words. Anyway, could it be possible it went down this way:
-Sidney calls Heather from home and he tells her he is leaving Tammy.
-Heather calls BW upset about the conversation.
-Sidney walks from his home to the pay phone (idk if it is close enough).
-He calls Heather from the pay phone and tells her he left the house/Tammy took his phone and keys, whatever, and he asks her to come pick him up from the pay phone.
-She agrees and picks him up.
-For whatever reason they go to PTL. Maybe to talk, maybe that was their spot, maybe Sidney just asked her to go there.
-When they get there, Sidney won't let her go (kidnapping charges), and shoots her, strangles her, however he did it.
-Tammy is at the house waiting on the phone call from Sidney.
-Once he does what he set out to do, he uses Heather's phone to call the pay phone (maybe for an alibi) for whatever reason.
-Then uses Heather's phone to call his own phone and tell Tammy to come pick him up.
-Sidney then destroys the phone (data ending).
-He waits for Tammy to get there and they dispose of Heather, or put her body in the truck to dispose of later.
-They then go home. This would be why the cameras caught the truck coming and going in such a short amount of time.
I'm just trying to make sense of anything that happened that night. Sorry if it makes no sense.