Don't expect to talk to them for hours on end. We usually talk to the whole family in the US for an hour but GS (two and a half) loses interest after a while.
Whilst we had the DGS with us, we skyped the other family - DGD also two years old - but with both kids fiddling with buttons and knocking ipads over all the time, we didn't actually get to say anything to each other. We kept it up for 5 minutes, which was enough for the little ones to recognise each other and say hello, but all the talking we wanted to get done had to be done subsequently by phone!
DGS is beginning to get the hang of it and will take the phone or tablet round the flat and show us things.
At five weeks you will be able to follow the little one's development with a weekly chat, and maybe starting them that early might mean that, in a year's time you can talk to him/her without getting seasick.
I would think, at that age, singing would be the best way of communicating. You can record yourself singing and send it to them, so that they can play it when the little one is most receptive.
Clapping, peep-bo, and little hand gestures, like "thumbs-up", can be introduced at a few months.
Good luck and congratulations on the new(ish) baby!