The issue sounds like a problem with VOIP. This is an issue that we are encountering in the alarm industry everyday. These systems are not designed to work with VOIP ( which stands for Voice Over IP, NOT Data Signal over IP).
The solution to this problem is a cellular module. These are available for most newer systems on the the market.
Long story short, VOIP is not the ""solution"" that AT&T, Vonage and other companies claim it is. You are better off without them in the long run. The money you save by switching to it, will be spent using an alternative method to make sure your Central Station gets your alarm signal.
We always recommend the SimonXT with SafeMart's revolutionary Plug and Protect. SafeMart has redefined how a security system is delivered and setup for our customers. Check it out at SafeMart's Plug and Protect.