There are three types of audio quality problems which can arise in VoIP. Excluded are all problems in which the callee side doesn't even ring.
1/ NAT issues:
When NAT gives you problems this typically results in ONE way, or NO way audio. This means on of the two sides, or both sides of the call cannot hear each other.
A common culprit is the router/modem which has SIP ALG activated.
To debug this:
* check if you can deactivate SIP ALG on your router/modem
* check if you can change NAT options on your phone
* check if you can change you're local sip port
* try and call from your network with a different phone , e.g. a softphone, to see if you can get to a situation which works, and then check the difference with what doesn't work.
2/ Parts of the call missing, cuts in the conversation, audio artifacts, bad sound quality
Typically all of these complaints boil down to a IP packets which are not travelling correctly from the phone to our machines.
This can have it's origin in every single component between the phone and our network.
A good approach is to :
1/ check if you have the problem with other phones too in the same network: if other phones don't have the problem, you might have a segment on your LAN which has bad cabling, or a phone which has issues. Change the position of your phone to be directly on the router/modem, to check if the problem is still there.
2/ check your available bandwidth, please be sure you have sufficient available upstream bandwidth, a call quickly takes 80kbit/s , if you have 256kbps line in upstream, do realize you cannot get +3 calls over that. You can check this with a tool like smokeping.
3/ make sure you don't have any bad shaping going on, in which you are not giving sufficient priority to anything VoIP. To be sure, you can assign the highest priority to anything which comes and goes to our servers. In many situations an older firewall can be the culprit.
4/ PBXes running on virtualized hardware can also be the origin of diminished quality.
5/ when in doubt, try with a softphone on your laptop, and connect it on different places on your network.