Who I have the same questioned this topic

I am unable to place calls in some areas of my place of work. This began about 15 months ago and I addressed it with Verizon but at the time, they couldn't figure it out. Someone recently mentioned to me that it may have to do with being on an extended network. Any thoughts on this? Also, one of my co-workers has Verizon and she has no problems making calls at all. I just bought a new iPhone and this hasn't helped the situation. 

Thanks in advance!