But not time dependant. I have 3cx current version running on Hyper-V. VERY low load. Voice quality varies greatly - sometimes it is horrific bad, most of the time quite good. Well, very good - it is a phone system, not a hifi setup. I seem to nail down the bad quality to people using VOIP on the other end, too (through a pbx) and / or mobile phones - but then also not in all cases. In these cases sound is seriously distorted - bad enough to make the call pretty unusable and I fall back to my mobile phone. Anyone an idea where to look next? I do not think this is a 3cx problem per se, but could also be something alone the line of my voip provider, or general interoperability. But any tips would be welcome. I do not think this is a performance issue on my side etc., because at the same time other calls work perfectly in parallel, and we can repro it (i.e. call alternatting numbers, it is always the same number with problems).