I have a D735 on 3CX hosted service.

After placing an incoming caller on hold, the phone plays a dial tone sound through the receiver earpiece. If the receiver is placed on hook, the sound continues to play through hands free speaker and cannot be turned off.

Dial tone sound plays continuously the entire time a call is on hold but is not audible to the incoming caller.

Navigating to other screens (settings, for example) mutes the sound temporarily but it resumes when returning to the main screen.

The issue is present on both 10.1.175.16 and 10.1.184.14 firmware versions.