iPhone XS Max keeps dropping calls.

See if the tips in the following articles are of any help.

If you can't make or receive calls on your iPhone - Apple Support

Does Your iPhone XS or XS Max Keep Dropping Calls? Here’s How to Fix It - iDropNews

How to Fix iPhone XS Max Keeps Dropping Calls

Dropped Calls

In telecommunications, the dropped-call rate (DCR) is the fraction of the telephone calls which, due to technical reasons, were cut off before the speaking parties had finished their conversational tone and before one of them had hung up (dropped calls). This fraction is usually measured as a percentage of all calls.

4 Reasons Your Phone Keeps Dropping Calls

1. Your Signal Is Weak.

2. It's Raining.

3. The Cell Tower Is Overloaded.

4. You've Misunderstood Your Signal Strength

Your phone also may be to blame for lousy service. Cheap phones often employ low-quality antennas that are vastly inferior to more expensive components, and battery quality impacts signal strength, too. That's a reason you should read user reviews before you buy a new handset. And if you still encounter problems, calling your carrier's tech support line may offer clues as to reasons for your dropped calls.

Make a call with Wi-Fi Calling - Apple Support

https://support.apple.com/en-in/HT203032

So the XS Max is a cheap phone? WOW!!!

Your phone also may be to blame for lousy service. Cheap phones often employ low-quality antennas that are vastly inferior to more expensive components, and battery quality impacts signal strength, too. That's a reason you should read user reviews before you buy a new handset.