I've maintained the same theory all along. I don't think it was planned. I think SM was out working that night at one of his jobs...and thought he'd call HE for a quickie or to see if he could slip back into her life. He called from the payphone because TM had his phone or he didn't want to leave a trail for her. I think that is the reason for the "he said he's leaving his wife and wants to see me" excuse. I don't think there was any hidden agenda at that point to do her harm.
But then when she perhaps balked or stalled...and he wasn't at the payphone when HE called back, she started calling his cell. I think at that point, when his cell phone was at home and HE was calling it, TM had it and got so mad she insisted on getting rid of the threat immediately. I believe once SM got home, she confronted him. He may have lied about making the initial call, or he may have come clean. But either way, I believe TM forced him to call HE back and say he'd meet her at PTL.
I believe TM rode in the backseat of the truck ducking down as they arrived. Once SM and TM arrived, he pulled up, HE jumped in the passenger side. As soon as the door closed, I think TM strangled her or attacked her. Remember, it needed to be a way to leave virtually no DNA evidence such as blood etc. I think that fits the bill unfortunately.
All of that is speculation of course, but it seems the most plausible explanation for the calls, timeline, lack of a "scene" at PTL etc. The only problem with my theory is...that means these two idiots were somehow able to hide her body so well that noone has been able to find it. That's the only part where I feel perhaps my theory falls apart since I don't think these two are so bright and thoughtful.
Just My opinion...