Every few calls I'm getting poor transmission / garbled conversation, I can just about understand them but barely. I have one handset a Yealink one that came with Bus router, it's happened a few times today from different numbers from various parts of the country. Bring back system X
This is the BT consumer forum which is for residential customers as you are on a business product your best trying the business forum https://business.forums.bt.com/
Yealink support is via the BT Business forum, nobody here uses those, they use BT Digital Voice which is a totally different system.
https://business.forums.bt.com/t5/Phone-system/bd-p/viop
There's no support
this is a residential customer help customer community forum your post does not go to BT