jbbts' suggestion solved the issue for me. We had the cell phone ringing for 30 seconds, reducing it to 20 seconds properly placed the call into the PBX's voicemail.
I did try Tech Electronics' suggestion, however, the user reported that they when they answere4d the call, they could hear the tone ringing instead of getting the caller. When they hung up the voicemail still correctly transferred to the PBX. I turned off the tone and it seemed to work as intended.