You could go with an IP base system with a dedicated PAP2T-NA ATA. I have Alarm Relay which charge $8.95 per month, paid one-year in advance. They now have the option of using their ARIM device (PAP2T-NA). At the time we signup with them, we had an unused PAP2T-NA ATA lying around and they allow us to swap ours for one of theirs at no charge. They charge $150 for their ARIM Device (which is only a PAP2T-NA with optimum secret configuration). Would suggest a backup for the Modem, Router and ARIM device in case of power failure.
There was one individual on this forum that was connected to their Central Station use a Obi and Goggle Voice. I'm not sure how reliable his setup was. You could do a search and message him to see how it is going. Cellular monitoring to me is more secure than even Land Line, since Land Line can be easily cut. The monitoring cost is higher than basic Land Line monitoring cost and the Cell Unit averages around $200.00.