I have installed Kamailio as sip server/registrar and using default config. I have this situation: mobile cleint using IMSdroid (extension 100) windows client using Linphone (extension 300)
Register satus: both clients register successfuly
Both clients get connected to kamailio using my Wireless WIFI modem.
I dial (from IMSdroid) to linphone: Both ring OK.
Audio heard in IMSdroid is OK. Audio heard in windows linphone is just wind sound and jitter (or train sound, when I speak louder it gets louder)
Video is ok in both sides (both clients show video clearly)
Both clients get connected to kamailio (public internet IP)
What the problem is from?
You say both softphones and Kamailio are using public IP addresses, if you don't force RTP relaying with rtpproxy (or other similar application), then the RTP streams are sent directly between the softphones.
It might be a bug of the softphone, a problem with the network or audio card, a misconfigured local firewall or maybe the carrier is blocking VoIP.
Try to use other softphone, like Jitsi or X-Lite and see if you get same problems.