Change Sim Card

Its nothing to do with the networks or branding.
Its your own personal security settings to safeguard your phones use should you lose it and someone else inserts their sim, it locks the phone from use.
Go to Menu/Settings/Phone/Phone management/security settings/phone & sim card
scroll down to lock if simcard changed, and change setting to "NO"
If you require a code for this and have not previously altered it, the default setting is normally 12345.
You will then be able to swap and use any sim cards presuming you phone is not locked to a network. If it is then you will only be able to use sim cards from your own network.

What happens if you put your girlfriend's sim into your phone?

It sounds as if your phone is locked to the SIM it came with.  If it was bought from a network supplier, that is probably the case.  You either need an unlock code from your supplier or you need a 3rd party service to supply you with an unlock code.

I buy my phones unlocked (yes they are more expensive that way) so I don't have much experience with who is good as a 3rd party supplier.

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isnt their any way possible of doing it myself.

I think he will need the provider to give him the security code that they used to replace 12345.

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