"Galwn ni heibio nes ymlaen."
Translation:We will call by later on.
From one American to another,
'Ffonio' means literally to phone, as in 'I will phone you later.'
'Galw' can mean a number of things (http://geiriadur.bangor.ac.uk/#galw&sln=cy), but in this instance, it means 'to call'. You can call someone in many ways - through the phone, paying a visit, etc.
Hope that helps! :)