If you have topped up from a credit or debit card within the last three months then you have free giffgaff to giffgaff calls - check by calling *100*1#
If you have free calls then these are free for up to 60 minutes, over 60 minutes then the call is chargeable. Please see the T&Cs
4.4. You can make free UK calls, texts, call forwarding and video calls to other Members subject to you topping-up or purchasing a Bundle using a credit/debit card or voucher once every 3 months. Such calls are at no extra cost for the first 60 minutes per call, after which the call will be charged using Charges applicable to standard non-giffgaff UK mobile pricing.