Skype on the phone

Hi all, I'm new at this so thanks for tuning in! I just signed up to an Garmin A50 but then realised how little actual talk time I get on my plan - the internet usage is pretty small too. I'm wondering whether its cheaper to put a Skype app on the phone and use that - does anyone know?? Does anyone know how much data a 5 min phone call take over the web (as in skype!).
I would love someone's help here!! thanks