S20+ Can't receive calls from my landline

[rjadev]

When I try to call my mobile from my landline, I get half a ring and then an engaged tone. My mobile won't ring, but the screen will flash a few times and I'll get exactly 3 missed calls.

Any idea why this is happening? Beginning to think it's a problem with my phone. I can receive other calls no issues, and I can call other numbers from my landline with no issues.