Each time I leave the country I sign up for IDT for international phone calls. There's a $2/month charge (you can cancel the service as soon as you return home if you don't plan to leave the country again in the near future) and that way you don't have to worry about running out of minutes or anything like that.

Per-minute charges are probably more than if you just buy a card when you get to Paris, but this way you know in advance that you have a service, and as you say you won't be using it much so the exact per-minute charges don't really matter as long as they're not astronomical (which they are not).

Details at http://www.idt.net/personal/cards/gc/ except that they don't actually list the per-minute costs from other countries (just the costs from the US to other countries) so you have to call and ask. I think last time I signed up it was 20c/minute back from the UK (I wasn't in France so don't know if it was the same or not).