Calls over WiFi straight to voicemail

Can't count how many pages of responses I've read on this issue, but haven't found a solution yet.

Problem: All incoming calls over WiFi go straight to voicemail. My Android devices do not have this problem, so I doubt it's a router issue.

Apps tested: Google Voice (shows missed call notification only), Fongo (rings when I have the app open, otherwise no notifications, call doesn't show up in call record, only get notified if voicemail is received). Have not tested FaceTime yet. May be able to do that today.

Network: Currently in a remote location with internet access only, no cell towers are accessible. Usually use Airplane Mode, but have tried without as well. As expected, no difference.

Phone setup:

iPhone 8, iOS 13.5.1

Latest version of above apps

Apps have all notifications turned on

Do Not disturb is turned off

Volume not muted (also tried unmute trick under Accessibility)

Silence unknown callers - off

Background App Refresh for above apps - on

Networking & Wireless (under Privacy>Location Services>System Services - off (have tried on as well)

Have also tried:

Reinstalling apps

Hard reboot (multiple times)

Reset network settings

Phone reset

Router: Asus AC1900P

SIP ALG - disabled (have also tried with enabled)

VOIP ports forwarded (have also tried without port forwarding)

Regardless of these settings, my Android devices work correctly

Any other ideas? I won't be back to a full-coverage location for a few weeks, so unfortunately I can't test over data. Not sure how long this has been happening, but I'm positive it was working before I upgraded to iOS 13 (approx December 2019).