Did it do that before you flashed? As far as I know the WM6 ROM wouldn't affect this.
It's probably either gonna be the PRL or the home SID that is your problem. I'm guessing you bought this phone used and it probably has the old users information still programmed in it.
What PRL version do you have on the phone? From the dialer screen, press menu, choose device information. Current Alltel PRL version is 30008.
For the home SID, from the dialer screen, dial ##778 and press send. On the EPST screen choose edit, and the password will be 000000. From the dropdown menu, choose Home SID. Make sure this is set to the proper SID from your local Alltel market. You can find this info here.
http://www.corolada.com/alltel/wiki/...rrent_SID_List
You should be able to fix both of these problems by dialing *228. Choose option 2 to update your PRL, and then call again and choose option 3 to update your phones programming information over the air.
Hope this helps, if not give me some more information and I'll help with what I can.