Don't bother. All of the cellular carriers are phasing out the analog networks, so it won't be long before it won't work at all. There was a big deal about this awhile back, because it's going to affect a bunch of OnStar subscribers. The carriers for OnStar are supposed to be discontinuing analog service on January 1, 2008; the FCC will no longer require analog cellular service as of February 18, 2008. AT&T is already decommissioning its TDMA analog network, and will drop all analog service ASAP; according to Wikipedia, Verizon has "noted" that it is "eager" to shut down its analog network ASAP, as well.
If you could get a conversion to digital, it might be worth it, but analog cellular technology is dead. It just hasn't finished twitching yet. (According to an '03 InfoWorld report I found, analog cellular network equipment was already out of production in '03.)