[inquisitor]

Thank you for your input, guys.

@maccoy
Neo2 sounds interesting. What about the data flatrate ("Web & Mail et TV 3G+ illimités 24h/24") - can you use it from a handset only or are you allowed to used it from a computer, too?

@andy
I also considerd callthrough, but I fear providers could block the dialin-numbers, like these wankers did here in Germany.
Are there VoIP-providers that assign local French DIDs so we could setup our own dialin-number on a FritzBox here in Germany?
Are there prepaid flatrate offers to French landlines without time slots?

solomo pro is a good idea for incoming calls, especially since I considered to let my mate's girlfriend switch to solomo pro anyway so he can call to France cheaper. I fear he will need a duoSIM phone.

VoIP-over-GSM/3G is too unreliable for us, since my mate is going to travel a lot and will often call from inside a car or a train, where data throughput is likely to become critical.