Mobile Operator Work Around
This is something most people have faced at one time in there lives is that you got a phone you like given to you or some such and you put your Sim card in and it will tell you it’s the wrong Sim card and you should put in the correct one, one way around this is to pay out to the mobile operator for a special code to open the phone to all networks but there is a cheaper way you can get a paper thin (except its chip) device that sits between the Sim Card and the mobile phone, what it does is block the Operator code in the Sim Card, So the phone doesn't know what network the Sim belongs too and continues to start up.
In this case I have an "i-SmartSim 2008" you can get yourself one here. Unfortunately you do have to make a small cut in your Sim Card as the little chip on the device is too thick otherwise (about the thickness of the Sim itself) but it doesn’t affect the Sim itself even if you choose to stop using the device or phone itself and the device is reusable!
Test Your Sony Ericsson Phone for Faults
In Many Sony Ericsson phones there is a secret service menu, now I am not held responsible for what you do in this menu, if you mess your phone up your on your own.
At the Standby screen enter one of these (some handsets use other versions of the code)
Normal Sony Ericsson Phones: > * < < * < *
P Series Sony Ericsson Phones: Jog dial Up > * > Down > Down > * > Down > *
At the Standby screen enter one of these (some handsets use other versions of the code)
Normal Sony Ericsson Phones: > * < < * < *
P Series Sony Ericsson Phones: Jog dial Up > * > Down > Down > * > Down > *