There are a number of members on Cocoontech who use or have used NextAlarm. I don't know if any of them have used the ABN, though.
My understanding of the ABN is that is converts the telephone line communications to data packets that are sent over your internet connection, and that's how it gets around the VoIP problems.
On the plus side for the ABN is that NextAlarm says it works with the Elk M1.
On the minus side, it is a proprietary interface box that only works with NextAlarm. So you are tying yourself to their monitoring service.
A more flexible solution would be to get yourself a M1XEP ethernet interface for the M1, and then you can use any monitoring service that supports IP monitoring and you won't need an ABN.
My preference would be to use IP and/or VoIP as a last resort. Both depend on the internet, which, in my opinion, just isn't reliable enough to provide for 24x7 security. Others here may disagree.
If you don't have a POTS line available, then the next best alternative is a cellular connection.
It all comes down to how certain you want to be that the central station will receive notification of an alarm condition.