'Do not disturb' doesn't (fully) work
If your iPhone also received a telegram call - it will pass that off to all your devices if you have Calls on Other Devices enabled.

I would review the DND settings on your phone (or other iOS devices) and then verify you are OK with the Calls on Other Devices settings.

This is found in the settings app > Phone section in the calls subsection towards the middle on iOS 13.

Either way, pretty peculiar failure - I wonder if it's just slow to fail or that your call times out by the time the iPhone wakes your iPad and the call goes to "voicemail" and wasn't quashed at all other than slow wakeup wasn't slow enough.