This happens almost all the time to me since a couple of days…
I start the call, it lasts 10 seconds or so, then the “call not sent” message appears in the middle of the screen, followed by “call ended” next to the phone number.
Switching off or rebooting does not change much. Even the latest OS update did not fix that.
Which sim card I use is not relevant since it affects both.
However, I have no problem sending SMS or Whats’app messages.
The only way I found to solve this (after trying many things) was to unselect the “ask everytime” setting for calls.
Then I am stuck using only one of the 2 Sim cards… Not very convenient for a double sim phone!