Hello, I have experience on integrated iOS Linphone for VoIP Calls.
I think this project need to add an better ProxySIP than flexisip. This will Kamailio.
In my experience Flexisip dont'n satisface complete and complex scenaries.
In my case, I worked Integrating Kamailio(ProxySIP)+Flexisip(PushNotify) <=> Asterisk(B2B).
May be for project that work with SMS, I will need to do a litle adjustment.
Same I don't have any problem on integration with FreeSWITCH. I too have experience In Deploy [login to view URL] (Multitenance Ip-PBX) and Free-Conference Systems based in this.
If you need more info don't excit to contact me.
Best regard,
Marcelo Bereterbide (Marhbere).