"Galwn ni heibio nes ymlaen."
Translation:We will call by later on.
I've never come across the expression 'I will call past' - perhaps 'I will call when passing' or something similar. The other option 'I will call by' makes more sense.
Just to make this clear for a confused American: "Ffonio" means to call someone on the phone. Does "galw" usually mean to pay someone a visit?
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! :)
What does "by"/"heibio" mean here? I don't understand it in either language...