I have a old Tesco phone (2010 ish) and I have just got a Tesco Moto G. I want to have my old number on the new sim/phone, my other sim is too old to just insert in to the new phone as it lacks lots of new features. On the Tesco website it says.......

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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.
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Are they saying I need to put the old sim in the new phone ? Or do I send the text from my old phone ?

Are you sure? Have you tried it?

Have they actually sent you a replacement SIM ? You don't do the SWAP until you have it. I think you need a MicroSIM for a Moto G? You then start the process to transfer your number across by sending the text from the old phone using the old SIM. You'll then be asked for the number of the New SIM (long number printed on it). Once it's done you can insert the new SIM in the new phone. It can take a while for your phone number to to be transferred to the new SIM - but usually under an hour.

My old Tesco phone had a SIM which was considerably larger than the one in the Moto G, though I must admit it was considerably older than 2010.

When I bought the Moto G, at a Tesco store, the salesman did the swap for me. Assuming you bought yours at a store, can't you ask them to do it?

The SIM also brought over £30 worth of PAYG credit from my old phone.

I swap a sim between phones without going to all the bother of sending texts and keying in sim serial numbers, imei's etc just to enable the sim to work on another phone.

Which is wonderful and all, except that in some cases you might need to swap SIMs to change form factor (without having to cut it) or to enable 4G.

Typing in a SIM serial number is not exactly fiendishly difficult and it only has to be done once.

Just ring them up and they sort it all out for you.

Tesco are helpful on the phone or in a Tesco phone shop. Live chat is a bit tick box. I swapped sims and to post it takes days. Better to go into a Tesco IMHO and have them swap it out. Doesn't take long to activate or didn't when I did it.

I have until recently had a Galaxy Ace phone but changed to an HTC Desire 300, and yes I didn't realise the SIM cards would be different sizes/

I've had to do the SWAP thing with Tesco due to not being able to put a big SIM in the HTC, that was used in the Galaxy Ace