Do you have the Apple iPhone? If so, this is a very common issue with iPhones when you change your mobile number and/or mobile phone carrier the Apple iMessage message service isn't updated with the newly transferred number.
Please follow the steps below to fix this issue.
Settings > FaceTime > FaceTime /Off
Settings > Messages > iMessage /Off
Now edit your iPhone to your new (i.e. transferred to giffgaff) number Settings > Phone > My Number
Restart your iPhone
Once it has restarted then please turn FaceTime and iMessage back on by following the steps below:
Settings >FaceTime >FaceTime / On
Settings >Messages >iMessage / On