Calls keep breaking up

[Only_me]

1. Calls keep breaking up irrespective of whether I make a call or others call me.

2. When others call me they often get the message 'It has not been possible to connect your call.

3. A visitor recently complained about how bad the o2 connection was when using her mobile data to connect to the internet. So bad that she gave up and used my wifi instead

Yet when I check the status on this website, it tells me the signal is good. 

It's not as if I live in a rural location, I'm in a city.

[MI5]

@Only_me 

Most likely network congestion.

As you have wifi, use wifi calling.

https://www.o2.co.uk/connectivity/wifi-and-4g-calling

Guide: How can I sort out my network issues? 

https://www.o2.co.uk/apps/my-network