Dave,
magic jack, vonage, and occasionally lines brought in through a channel bank have always caused problems
for alarms. Usually it's voltage and line current issues.
Even if you got it working it would still be dependent on the ISP being up.
That being the case you might as well just put in a data module on the alarm, (almost all of them offer it),
and have it transmit over the internet.
A lot more features and he could also interface it to a smart phone.