I have an unlocked Nokia 3100 bought on ebay. The opening screen displays ATT.

I have used it in Russia with a Russia sim, and US sims from Cingular. It appears to be unlocked.

I bought the phone so I could use it in the UK. The UK band is 1800 mhg and the 3100 supports that band. In the UK I have tried it with a O2 Sim and a Tesco Sim (Tesco gets network service via O2). It will not locate service from either. It will locate and display the Orange and T-Mobile networks on the manual 'network select'. It will connect to the UK Tesco network from the US (via Cingular roaming).

I took it to an O2 store in UK and they tried the Tesco sim on an in-store phone successfully, so I think the sim is ok. I have an old Philips Saavy dual band and the sim works on it. (The O2 sim was originally used in the Philips saavy, the Tesco sim tried unsuccessfully in the Nokia 3100 then it worked in the Philips.

What would be the reason this phone will not get the O2 signal. Is it not unlocked (properly - completely)? Is there a degree of 'unlock'? Is it a software/hardware problem? Neither the O2 or Tesco customer service line was able to help.

How does a cell phone identify a service network? Can you point me to a document?

If I get it fixed I will not be able to test it til my next trip next fall.

Thanks