A few things to consider:
1. She could have been killed by someone known to her who was with her or followed her. There is ample motive when it comes to family matters, for example.
2. She could have been placed in the trunk facing inwards and was unable to turn over to get to a release latch. Hindered movement may have also contributed to how long it took to get her phone into a position where she could use it. She was not a petite woman.
3. It takes more than gun residue on the hands to determine suicide.
4. For all we know, a possible perp made a mistake in not thinking about her having a phone on her person, or may not have been able to detect it if he hurriedly patted her down. Mistakes are how they get caught, after all.
5. The police calling her phone may actually have been what alerted him and made him pull over and shoot her to death. I find that to be an incredibly risky move by LE. Not wanting to be ID'd would be of utmost importance to a perp like this if it was random. Why didn't he kill her earlier? Perhaps he planned to ransom her, was instructed to deliver her alive, or felt like raping her once he reached his destination, wherever that was.