How Did My Dad Hear My Call If He Wasn’t on the Phone?

Have you considered that maybe they had a Bluetooth speaker or some other device connected? That could definitely explain how your dad heard you even though he wasn't on his phone.

It sounds like what happened is probably an acoustic glitch rather than anything to do with the call itself. Since your dad was down the hall and your mom was on speaker, it's likely that he could hear you because of the way sound traveled. Don't forget that with 5G technology, calls are encrypted, so some sort of mixing of signals seems unlikely. You may have experienced a strange combination of sound and noise cancellation, which can occasionally happen in these scenarios.

You might want to check the call log to see if both phones actually registered the call. Sometimes, a glitch can occur if they tried to merge calls or something along those lines. Also, see if they are logged into the same Samsung account; there's a feature where you can take and make calls on other devices linked to that account.

By the way, are they users of T-Mobile's DIGITS service? If so, that could allow multiple devices to ring simultaneously, which might explain this whole situation. Taking a step back and not worrying about it may be the easiest! It could just be one of those random tech glitches.