Cingular Ericsson K750i and P910 Help

About 3 days ago mine and my fiances ericsson phones on the Cigular network just stopped working. We seem to be connected to the network, but everytime we both try to make a call we get a "Network Failure" or "Network Busy" message.

Both phones were purchased in the Middle East not the US. Both worked fine with WAP and MMS for over 3 months. Both phones are running off of factory firmware.

We called Cingular and ofcourse and we got the: "The reason is your phone". But whats the point of two triband GSM phones, if I can't get it to work in the US. I'd at least like to know why this is happening.

Thanks everyone for responses

Edit: We cannot recieve incomming calls as well. And our SIM cards work inside other phones. So guessing its a phone issue.

Update

I upgraded firmware on the K750i, and to my supprise that worked. I will try and do the same with P910 when I can find my cable for that phone .

You will be able to use both of these phones anywhere Cingular has 1900mhz towers, or roam on T-Mobile, who only uses 1900mhz. The K750i has a 900mhz band instead of 850mhz, so there might be some areas it won't get reception if 850mhz is the only available service. The P910 was a good choice, it has both bands that Cingular uses, 850/1900 so hopefully upgrading the firmware will make it start working again. Good luck!

Thanks Brad729