I'm not an expert on the Oooma, but ours is physically plugged into the regular landline jack so that our entire phone system works with the Oooma and so we can make calls on the one non-cordless phone we own. I actually think that phone is a piece of junk and am planning to donate it to someone who needs *something* (thinking battered women's shelter). Anyway, my point is I think the Ooma might work for an alarm system. Try googling it perhaps? Ooma has a website and I'm sure this has come up before.
Sound quality probably depends on many factors. The MagicJack just didn't cut it for us, but it may work very well for others. The Oooma works even BETTER than our previous landline and it is the exact same phone system with all three scenarios. We got the phone system on Black Friday a few years ago....one of the ones that only the main base station plugs into the landline jack and the rest of the handsets just need power. Technically, we could plug it into the Ooma instead and never need to plug anything into the landline jacks, but we prefer the location to be where we can see the message light blinking and prefer to use the answering system on our phone system versus the Ooma system. It is merely an out of sight, out of mind issue. Ooma can handle all of that and way more. In fact, Ooma voice mail kicked in the other day when I was chatting on the phone with a friend I hadn't talked to in ages and I didn't want to click over to call waiting (I could see who the caller was). Oooma took the message for me in that case and I got an email with the voice-encoded message attached.
There are lots of choices available to suit many people.
We happened to try three and really, really love the Ooma the best!
[As stated above, cell phones are very inconsistent in our immediate area, so going all cell was/is not an option for us. Besides, I'm not sure we would do that anyway. Who handles all the "family" calls? I'm home more (am available to take calls more often), but I ignore the phone when I am engrossed in something else and I would never carry my cell around with me as I move all over the house and yard during each day.]