How can I improve Wi-Fi Call Quality?

[DrMopp]

BT Infinity, SmartHub, Upload Speed of ~20Mbps Download of ~72Mbps Wi-fi signal strength shows as 'Excellent' on mobile with Link Speed of 300Mbps. Wi-fi calls always connect but there is often a problem with the called party saying the call is 'muffled' or dropping out, although I can generally hear them clearly.  Anyone any ideas before I go back to the landline? 

[Frodok]

When I was in Brazil, I faced the same issues. I was in San Paulo and it's a big city. You can see there that its population is ever-growing. But the situation with the Internet isn't anyhow decent.