Hi there! My name is***** am a computer professional with over 15 years of experience with both Mac and Windows computers, software, phones, tablets, e-readers, and printers. It would be my pleasure to assist you with this. Please try the following from the phone for which the calls are being forwarded. In order t best help I will need to ask a few questions. Do all calls go straight to voicemail, or is it only outside calls? Can someone call form another extension on the phone system and not be sent to voice mail? Is Forward on Busy turned on for the extension? Is DND on Busy turned on for the extension? What is the model of the phone?