Still no luck.
The biggest issue that from time to time we can hear each other perfectly (I have to say maybe 2 calls from 10).
So it seems like not the network/firewall issue.
We can make an outgoing calls through SIP with no problems at all. But when we got an incoming call through SIP we can't hear other side, but they can hear us perfectly.
In my experience, if it has worked perfectly - you have it set up correctly, so its probably not a ksu issue, leaving the possibilities to a. internal network traffic, router/firewall traffic speed and load, internet traffic/load and provider troubles. If you have a router that has the capability of giving you traffic load meters/graphs that would give you some ideas that way.
The second quote above - you can not hear the caller but they can hear you - tells me that the problem lies in download internet/network traffic speed not upload. (Upload being your voice sent up to the caller) Possible any site user is streaming youtube vids or their favorite internet radio station - and they leave for the day allowing the SIP traffic? Is it possible to disconnect the user internet for 5 minutes to test without their load?