I feel like the core product is pretty easily replicated with Freeswitch and tailored with a little configuration. Eg: Ring a phone for 30 seconds (so as not to leave a message accidentally), if answered read back the issue (using text to speech), otherwise send the phone a text and <insert whatever else you want to do>.

Having set systems like this up for numerous clients, the latter bit is usually a case of waiting a few minutes for the on call person to either accept the alert, or failing that move onto whoever the fallback is for that person.