iPhone11 drops unknown calls

OK, then scam robocalls clearly aren't the issue. Are these dropped calls showing up in Recents in the Phone app? If not, then they're probably not making it through and that would be something that you'd need to take up with your cell carrier.

I doubt that's the issue, though, given what you've said about the issue. Some things to try:

• if you haven't done so already, try restarting your iPhone.
• Reset your iPhone's settings to default by going to Settings > General > Transfer or reset iPhone > Reset > Reset All Settings. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
• Finally, try a factory reset.

Perhaps one of those steps will help. If not, someone else here may have additional suggestions.

Regards.

.

Perhaps you have Silence Unknown callers turned on.

Settings > Phone > Silence Unknown Callers

I can't be sure, but at least some of these may be scam calls. Many scammers and spammers robo-call thousands of numbers sequentially just to verify that a certain number is working and will immediately disconnect if the call is answered. They then put that phone number of a "known good" list for later actual scam calls.

Whether this is the case with your calls I can't say. Can you get someone whose number isn't in your Contacts to call you and see if the call comes through or still is dropped?

Regards.