If I used my personal cell phone to conduct business for my employer and they pay partial for the bill can they keep my phone upon termination?

Assuming it's your phone and your contract does not state anything else, then

No

they cannot. How would that work out? I regularly use my private car for company trips and get reimbursed for that. Doesn't mean the company gets to keep my car when I leave. Anybody suggesting that would be considered out of their mind.