You can troubleshoot this through the process of elimination.
Don’t make any calls from home for a day or two. That’ll tell you if it’s something localized to your home/location or if it’s specific to your phone/contract.
Disable all extra technology like Bluetooth. That’ll tell you if it’s a Bluetooth or other connection. Ideally, turn off all extra connections beyond cellular. You could also try voice/video calls vs. phone data - I doubt this would matter, but it’s always the last thing that you check.
Break your house into a few different zones. If they comment on a conversation held it in one part of the home but not another, that’ll give you an idea if it’s location specific.
Probably not the most helpful tip, but how about taking this opportunity to learn another language?