If this is just for outbound calls from the alarm system, a separate VOIP box would be very inexpensive. I use VOIP.MS which is prepaid. Even using their premium tier outbound service the cost is one cent per minute. The only monthly cost is for inbound phone numbers ($1 per month), which you may not even need.

A $25 prepaid account would last a long, long time in such a configuration. Obihai makes several boxes. I prefer the OBi202 as it is fully featured. Configuration is a bit technical but once set up it just sits there and works.

Only time I have 'phone trouble' is when my main Internet service goes down. When the Internet comes back, the Obi recovers without me touching it.