I am having a problem forwarding to verizon cell phones vs sprint or any other carriers, the result is the cell phone rings 3 times then hangs up, all other cell phones ring normal then my call rolls to the cells voice mail. I have been troubleshooting it from different areas of the city and can forward to verizon in different areas, i am starting to think it is a call type or a specific cell tower. the pbx is an 81c 3.0. Any one have this issue before?