I once worked in an inbound sales call center. Call the 1-800 number from a TV commercial, and you talk to someone like me. We were in the US with decent telephone infrastructure, but every call transfer or conference call would degrade the call quality, and it often wasn't good in the first place. My layman's understanding is that telephone audio is already not very good by design due to varying capabilities left over from now-obsolescent systems still in place. Something about the networks made it worse.