Calling between two roaming SIMs might be a viable solution as you really want to avoid paying any incoming call charge or roaming charge.
France - UK Calling Another GeoSIM Mobile £0.38 ($0.72)
UK - France Calling Another GeoSIM Mobile £0.32 ($0.60)
Receiving calls is free in both countries. 6 second billing.
If either of you change countries, then you are both always only calling the UK number.
details on
www.globalsimcard.co.uk
"Other SIMs are available"