Do you have airtime credit? Airtime credit is required for international calls and texts as goodybags can only be used within the UK when roaming in the EU(selected countries). This also applies when your calling any 055, 056, 070, 076, 084, 087, 090, 091, 098, 118.
Complete international pricing can be found here http://giffgaff.com/index/international
If you do have airtime credit please check the number is in the correct format along with the country code. Please check out this https://community.giffgaff.com/t5/Solve-your-questions/How-to-call-foreign-phone-numbers/ta-p/351653... guide on how to call foreign numbers
In the UK, goodybags include calls and texts to UK landline numbers starting 01, 02 or 03, UK mobile numbers starting 071, 072, 073, 074, 075, 077, 078 or 079, voicemail (443) and mobile data.
Elsewhere in the EU/EEA with Roam Like At Home enabled your goodybag also includes calls and texts to any EU/EEA landline or mobile number (conditions apply)
Goodybags never include:
for which you need PAYG credit
This guide should help you understand how you will be charged for a call, text or mobile data wherever you are in the world.