Kamailio and android SIP

Question

I'm using Kamailio and OpnVPN on a server, and CSipSimple on android devices.,
The problem is that with some devices this works and with other devices it doesn't work.
When one of the devices is less known device (Alcatel) - it works.
When both devices are well known (LG G2, Nexus 5, Galaxy S5) - it doesn't work.
By doesn't work I mean that the session is created (when dialing, there's a ring sound on the other side and the other side can respond), but no voice is transferred between the devices (no one can hear the other).
Does it happen because of the devices, the SIP clients, the VPN or the SIP server?
Thanks in advance.

Edit

I found that it is probably related to KitKat (ie, it doesn't work with 2 devices that have android KitKat) but I don't know why and how to fix it.

Answer 1

Finally solved it.
We found that it is related to Android KitKat.
Instead of using CSipSimple we are using Sipdroid now.
We downloaded the code of Sipdroid, compiled it and played with codecs. We found that a specific codec caused the problem and when we removed it - the voice started working.
Maybe there's also problem with Kamailio that stops audio from KitKat devices.