I did the below yesterday to port over my old pre-3G SIM phone number to my new phone (the SIM was too old to just insert). Both are the Tesco network........

---------------------------------------
Swap your SIM
Can you send Text messages from your original SIM?

If so, you can swap your SIM by text:

Text SWAP to 23424 from your original SIM card
You'll get a text telling you that your number will be transferred. You'll then be asked for the serial number on your new SIM card
Reply to the text with your new SIM serial number (this will be either a 13 or 19 digit number)
You'll get a text telling you to place your new SIM into your phone and the transfer will be completed within 24 hours.
---------------------------------------------

Then just as instructed today it came up with no signal/bad sim on the old phone so I then put the new SIM in to the new phone. It comes up with Tesco fine, but the phone number showed is still my new one (not the one I wanted transferred from my old phone).

Shouldn't this now be my old number if everything went well ?

No it won't change even after a successful swap.

I manually edit mine using an old Nokia 6310i.

But what have I just done all that for then ? Is it meant to work ?

If the new SIM receives calls and show the new number that you swapped onto it on outbound caller ID it is working fine even if the SIM is displaying the now defunct number.

Ah I see. Yes, I got a text today and it was for the old number. It seems like you say, it is just displaying the wrong number to me. I have the app "my number" installed as a widget and I can't seem to change it. Do you know any other phones that can change the SIM number (other than Nokia 6310i) ? I have a old Sony Mini Xperia and Alcatel phone lying around that I could try. I might even have a Nokia. How do I change it from them ? Thanks.

Sorry it wasn't my 6310i that was able to do, it was my even older nk702/6130 that was.

I think there maybe an app for Android that can do it but it requires root.

I think BlackBerry handsets can also, certainly those from the past 5 years.