New phone for VERY old plan?

Hello!

I have a Cingular GAIT plan which is approximately 7 years old. My Sony T62 phone broke in October 2005 and I replaced it with a Nokia 6102 from Let's Talk. I never told Cingular, just popped in my old SIM card and away I went.

Well, this week my Nokia broke and I needed a phone ASAP so I went to a Cingular store and bought a black RAZR at full price. The rep told me I needed a new SIM card if I wanted to access all the towers, but that they didn't have one in the store and to call customer service and have them send me one.

When I called customer service today, they said I would need a new plan to use this phone, and if I tried to use it with my old SIM card or a new one it would result in billing errors as the phone would incorrectly charge me for roaming.

The kicker: Cingular cancelled service in my area (NE Kansas) last September. I was not removed because I have my old number from where I previously lived, and all my bills have shown my calls to be in area due to the GAIT plan. I get good reception, but Cingular does not offer new plans in my area because the tower is now owned by a different company. My only other service provider options are Cell One or Alltel, and I REALLY don't want to switch providers. I travel throughout the rural midwest, and Cingular has always been great.

This is a long winded way to ask if anyone knows whether this phone will really cause a problem with my billing if I leave it on my old SIM card, or if my current plan will allow for the newer SIM card. My Nokia 6102 never caused any billing problems, but I never switched to the newer SIM card it came with. My RAZR appears to be working fine as well, but I won't get my next bill until after my 30 day return period has expired, and I really don't want to be out $220. Thanks!

Shayna