Call waiting issue after One Ui 6 update

[Easty]

So when I'm on a call if another call comes calling waiting doesn't always activate it cancels the call I'm on then the new call rings ? Doesn't happen everytime but still annoying as I get cut off mid call without any warning then new call comes in ? 

I've checked call waiting is activated, even turned it off back on , restarted phone , any ideas ?

[-Robot-]

Call waiting is usually managed by the Network Provider, have you spoken to them?

[Easty]

No , it's only since update , vodaphone are ***** to get any sense out of

[Easty]

Forgot to say I've 2 sims in phone

[Easty]

Ive turned off dual sim always on as 2nd sim is a back up for data when sim 1 has none , see if it makes any difference