Yes when you port the number into giffgaff any active allowances e.g airtime credit will remain intact the only thing that will change is your number.
A (Porting Authorisation Code) is required to transfer a mobile number from one UK network to a different UK network. You can transfer any UK mobile phone number to giffgaff, provided you have fulfilled any contractual agreement you may have with your previous service provider.
If you're still locked into a contract, it may be possible for you to leave it early although you may be charged a fee - typically this would be your remaining line rental.
The PAC Code you will receive from your previous provider will in a format of ABC123456.
Once you've got your PAC the next step is to fill out the Transfer your number form here http://www.giffgaff.com/support/transfer
with the PAC Code and the number you want to keep.
Your preferred transfer date must be within the next 14 days, but cannot be the same day that you submit your request. You can't transfer your number over the weekend or on a Bank Holiday.
If you want to transfer your number the next day you MUST submit your PAC before 17:00 (5 p.m.) on the day of your request. Please see the table below:
Time / Day of request Earliest transfer day
Between 17:01 on Friday and 17:00 on Monday Tuesday
Between 17:01 on Monday and 17:00 on Tuesday Wednesday
Between 17:01 on Tuesday and 17:00 on Wednesday Thursday
Between 17:01 on Wednesday and 17:00 on Thursday Friday
Between 17:01 on Thursday and 17:00 on Friday Monday
PLEASE NOTE save your contacts to phone memory or back them up before you transfer your number as the contacts from your old SIM will not be transferred over.
On the transfer date you will experience a loss of service at some stage between the hours of approximately 9 a.m. and 9 p.m. Typically this is no more than a few hours but may take all day, so please make plans so you're not caught out.
The transfer is a manual process and the start time is dependent on a number of factors including the time your old service provider releases your number, so giffgaff cannot say what time your transfer will start or how long you will be without service.