Getting 'Busy' tone when the call ends

Whether it’s coming from outside, or an internal extension, when the other person hangs up, the phone remaining on the line gets the “busy” tone. It wasn’t like this before, it’s new. And I sincerely can’t think of what changed in the interim.

Interestingly, for some reason, this only occurs when the phone is in handset mode, not intercom/speaker mode. If the remote call hangs up while the phone is in handset mode, I get a busy tone. If they hang up while it’s in Speaker mode, the call just ends. Doesn’t matter who initiated the call. We have Grandstream’s GXP2160s.