UK numbers that start with 03 are uk-wide geographic numbers. They are regulated by Ofcom, and cost no more than calls to ordinary landline numbers that start with 01 or 02. By law, calls to 0370 numbers must be included in any free or bundled minutes that you have.
On GiffGafff, as long as you stay within your goodybag plan, you won’t get charged any extra for calling an 0370 number. If you do go over your goodybag allowance you have 2 options:
1. You can choose to start your next monthly plan early.
2. You can pay-as-you-go until your new monthly plan starts.
The pay-as-you-go option means that you will be charged the standard rate for all calls. The standard call cost for 0370 numbers on GiffGaff is 15p per minute.