Unlock Code

[Refeat]

Can someone help me out.

I have a Sony Ericsson Z600 that I want to put my O2 sim into, but it's asking for an unlock code.

I bought the phone on ebay in 2005 and I'm not aware if it is locked to another network. I know there were O2 booklets in the box it came with.

Surely if it is an O2 phone I shouldn't need an unlock code. If not then how do I proceed?

[MI5]

You can buy a cutter which is a bit like a hole punch but unless you are doing many, it’s cheaper to pay the usual charge of £1 for the market stall to do it.
I wouldn’t recommend using scissors and guesswork though.

[Refeat · Accepted solution]

Just an update and conclusion (of sorts).

 I purchased the DC111 cable to factory reset the old Sony Ericsson phone. However, the software CD it came with called Mobtime required Internet Explorer 4 and I couldn't find a decent version online. The Sony Ericsson suite didn't work with the phone either. 

 So then I bought a hole-punch type device that MI5 above was talking about to change the old Sim card into a Micro-Sim to fit the new Nokia 3310. The hole-punch worked, however, the Sim card did not. Another waste of money.

 So in the end we managed to get the contacts from the sim card transferred via bluetooth connection. I'm not entirely sure of the process but another family member managed to do it. 

Upshot is that the new Nokia 3310 is the replacement phone, although the reception on it doesn't seem that good. Not too many bars compared to the previous phone. I'm not sure what can be done about that (perhaps nothing?) it's an O2 phone sim fwiw.

[MI5]

That's all the fun of old phones
Reception may not be as good on new phones on 3g if you are comparing to an older 2g only phone.You can try selecting 2g only in your network settings to improve signal reception.