newbie, sch-a650 activation

I curently use a prepaid cell phone service, which uses the verizon network.
I bought a used samsung sch-a650. This phone is for verizon.
When I took it to my service provider, he told me that he could'nt
activate because it was programmed for alltel. This confuses me.
The a650 is a verizon phone, how could it be using alltel? and how
can i get it to accept verizon?
My wife uses this same phone currently on this plan.

thanks in advance for any info on this subject. I have very limited
knowlege about how these networks work.

bert