Sounds like Handsfree (not to be confused with Speakerphone) is enabled on the phone. On the 3102 phone, it's a hard button that can be pressed on/off, but on the 3101 phone, it's not a button. This feature allows Internal callers (by default..also able to be allowed to external callers, however I don't know why anyone would!?!) to this phone, to beep once, then open up the speakerphone, and the user can respond 'handsfree'.
I don't recall if NBX has a feature code assigned to this feature, but VCX does.
I'd probably just do a Eeprom reset on the phone, to set it back to factory default. You'd have to reboot the phone, and press the white dot 'Program' button within 30 seconds of reboot, then use the arrows to scroll down through the menus to Advanced settings, and find Eeprom reset to default, and choose that. *Note- that will reset all values of the phone, so if it's Ethernet Only, it's fine, but if the phone is using Standard IP, it will also clear the IP address of the phone*