If you are a giffgaff member already and have a giffgaff number but you want to change that number to a different one using the same sim and forwarding on your credit and goodybag due to nuisance calls / text etc, then check out the below.
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On some smart phones there are settings where you can block numbers and text, there's also Apps out there that can also block certain numbers and text, if you just want to ignore all that and just change your number keeping the same credit / goodybag / account as the one your using now with the same sim then just follow the below, >>>
This link will explain everything you need to know / do , https://aroundandroid.com/call-blockers-for-android/
( Windows )
1, Navigate to Settings. ...
2, Hit Call+SMS Filter.
4, Turn on the toggle to block numbers. ...
5, Go to your call history by tapping the Phone app. ...
6, Long tap the number you want to block. ...
7, Hit Block Number.
8, Tap Ok.