IIRC, the cell phone is of an analog design, yes? Likely a motorola type, like the old bag phones.
The only way to get that to work with your plan is to have it cloned to a current portable cell phone, and even then it's hit an miss, especially if you only have digital service, like GSM, or a digital only dual band fron verizon. If you have an older triband from verizon for example, then it would work great after you clone it to that phone.
As far as getting it cloned... PM me. It will run you about 125 bucks plus shipping, IIRC. I'd have to call the guy who's done a few cloning jobs for me and ask him if he can clone that particular car phone. I'd need a model number and manufacturer (likely its a motorola transceiver).