I am running Asterisk 220.127.116.11 currently and we have the follow-me module configured on our desk extensions so if we aren’t at our desks the calls will forward to our cell phones. When this occurs the volume is so low in about 2 out of 3 calls that I can’t hear the person calling but they can hear me. We are a SIP only box with now analog trunks so the normal troubleshooting I’ve been able to find regarding adjusting gain on the ZAP card doesn’t apply and I can’t find any other suggestions of what to look at. Calls direct to cell phones work fine, calls answered on the desk phone work fine as well. It’s just the forwarded calls that experience this volume reduction.
It wil end up in a normal bridge, so one wouldn’t expect any manipulation of the audio other than transcoding. Maybe you are ending up with a different codec, and either Asterisk’s transcoder isn’t preserving the level, or the phone doesn’t have the same levels for different codecs.
Your version of Asterisk is a long way behind the current (and probably final) 18.104.22.168 version, so there will be a lot of bugs in that version, that are now fixed.