3rd party app like zalo and messenger don't display incomming call

[supnep]

My Samsung watch 6 do not display notification of incoming calls and the option to accept/reject call from 3rd party apps like Zalo and Messenger while it is still able to display calls and options from the system calling app or Grab driver's call

[VerbumVeritum]

It might have something to do with the fact that those apps you mentioned use VOIP service (internet calls) as opposed to an actual cellular network.