So that we can determine what the issue is regarding your sound quality issue, please could you advise on the regularity of this problem and also ask if you also have this issue on your landline telephone when off-shift. The best way to accurately diagnose the issue is to ask if the telephone can be used normally direct to the telephone socket with no other equipment attached to it e.g microfilters, double adapters etc and make a call to see if the issue persists.
If the call quality issue remains, your telecoms provider needs to give you extra support in diagnosing the issue. Buzzing on the line is usually an outside line fault known as 'battery-contact' but classed as 'quality of service issue, your telephone provider will always ask for another telephone device to be tested before escalating as standard practice to rule out equipment being the culprit, however, if the fault still exists, a telecoms engineer may need to attend the outside line and some rare instances a specialist REIN engineer.
Could you please follow these diagnostic steps and should the issue persists, escalate the issue with your telecoms provider.