Hi, have you been using the saisim in another phone
or is it a new sim and you've performed a sim swap?
. Have you switched your phone off and on?
can you see your mobile number and balance /goodybag here
At the top of the phone
Do you see signal bars
giffgaff or O2-UK logo
(3G, 4G, H, E, gprs)
From your phone keypad dial
Star 100 star 7 hash
and press call
Do the same with *100#
What word or words do you see on the phone screen?
What do you hear exactly when you call your number from another phone?
Do you have another unactivated giffgaff sim card?