Unlocked Samsung E-715 No Longer works with Cingular

I noticed this past weekend that my unlocked Samsung E-715 that I bought
when I was with T-Mobile starting losing connections. Now I can not pick up
a tower anywhere. The phone is unlocked and I have been using it for
months. I am on a family share plan and my brother has the same phone with
the exact same problem, same timing.

Does anybody know why and what can be done.
My simcard works because I tried it out in a Cingular phone with no
problems. The problem lies with my unlocked phone which seems to locked out
again. I wonder if there was a frequency change.

My phone is 900/1900 and Cingular is known for 850/1900 but if they moved
all their towers last weekend to a new frequency "all to 850" then there's
nothing I can do.