Changing the number on a S/H mobile phone

[Old Sparky]

I'm told that, if a new SIM card is put in a second hand mobile phone, this will automatically generate a new mobile number for use with the said phone, and that the previous number and any other data relative to it's use -i.e. incoming messages & calls to the previous user will be erased.  This makes sense to me, since data is presumably stored on the SIM card (btw for what does SIM stand?), but can someone confirm whether or not I've been correctly informed.  Not being a mobile phone owner or user doesn't help, so sorry if this is asilly question.  Further to this does a second hand phone have to be unlocked before it can be used again?

[ppppenguin]

The phone number belongs to the SIM, not the phone. SIM stands for Subscriber Identity Module. When you buy a new SIM you are given a phone number with it.

Phone numbers can be stored on the SIM, the phone or both. Text messages, call records etc are normally stored on the phone, not the SIM. if you're re-using an old phone then google how to do a full erase and reset. This is for smartphones but there will be instructions available for just about anything online. https://www.cnet.com/uk/how-to/how-to-wi...e-selling/

If your phone is locked to a particular network and you want to use it on a different network you will need to unlock it. http://www.moneysavingexpert.com/phones/...bile-phone

If your phone has been barred, usually because it's been stolen, then it's illegal to attempt to remove this.

[Nick]

Agree with everything Jeffrey says - many phones are locked to a particular network because under the deal between the phone manufacturer and the network, the network can offer the phone cheaper that way.

If you then try to use a locked phone with a SIM car for a different network, it won't work - you have to get the phone "unlocked" first. This may or may not be a legal thing to do, as explained above.

e.g. you buy a phone locked to O2. You put in a new Virgin pay-as-you-go SIM - the new SIM will not register to the Virgin (or any) network until the phone is unlocked. If you use a pay-as-you-go O2 SIM, it will work (same network as the one the phone is locked to).

Basically, you should also do a "factory reset" on the phone to erase all non-operating-system data on the phone, i.e. all pictures, SMS messages, email, yada yada yada...

[ppppenguin]

If you then try to use a locked phone with a SIM car for a different network, it won't work - you have to get the phone "unlocked" first. This may or may not be a legal thing to do, as explained above.

Just to clarify. Unlocking a phone that locked to a specific network is always legal. A phone that's barred won't work on any networks and is effectively a brick. I'm sure there are illegal methods to unbar a phone but GVR is not the place to discuss them.

[Mike Watterson]

Yes, mobile phones have IMEI as identity, The number is from the SIM. Vodafone gave me a free replacement on my main phone* and transferred the call credit as I was getting nuisance SMS messages. I didn't reset anything.
I also have a £15 "smart watch" which unlike the Apple Watch (which needs an iPhone), works as a notification or handsfree voice via bluetooth (on any phone), but is also a GSM (only!) phone. It's awkward to put call credit into, so I popped the SIM into my "real" phone for that. I use it to receive SMS from certain web service. Fairly rubbish camera, but excellent voice dictation. It ALSO takes a micro SD card!
Using it to record a conversation may be illegal if the other party isn't notified.

The only exception is USA CDMA phones, the number is locked to phone. Unsurprisingly Apple wants to ditch the physical SIM. That would be evil.

(*I have a 3G WiFi dongle which takes SIM and works like a smartphone hotspot, 2 x smartphones and watch phone in use. I do still have a Nokia in a drawer that I used the serial port cable on a server and wrote a program to control / interrogate database via whitelisted SMS numbers (2003), not a safe occupation now as numbers can be spoofed, and indeed a phone base station faked for under $1500 for Man in the Middle Attacks)