Are you receiving standard green texts ok?
Text the word NUMBER to 2020 and also to 43430
Do you receive replies from those two?
(if you can receive green texts ok I'll give you some steps to complete on your iPhone)
It is important to contact an agent to report the incomplete transfer, so please follow the advice in my first post and contact an agent via another account
Once you're logged into your temporary second account, use this link to contact an agent https://support2.giffgaff.com/app/ask/Number-transfers/Partial-or-no-service-after-transfer/form/
Don't forget to include some details to prove you're the owner of the active account (again described in my first post)
Regarding the sim serial nunber
Rather than taking the sim out of your phone, you can retrieve that number by...
Going to settings
Look at the number on ICCID
there should be 19 digits