Unable to receive calls after making outbound call

I have a number of extensions on our system (Asterisk 1.6.2 with FreePBX) that seemingly have DND enabled after making an outgoing call. More specifically, after they hang up from their outgoing call any incoming calls to that extension go directly to their voicemail. If they use *76 (toggle DND) it says “Do not Disturb enabled” and then they use *76 again to disable and they can receive incoming calls again, until they make another outbound call.

I am fairly new to asterisk so any assistance as to where to look or something to check when it happens, etc would be greatly appreciated.