Given the problems detailed, I suspect your issues may have more to do with the stability of your network connection combined with some quirkiness of your home network equipment. If this is the case, then different providers may not be the magic bullet solution that you're hoping for. Some hardware and provider configurations from different vendors are more tolerant to the more problematic locations than others, but the best resolution is correcting the issues that cause the outages in the first place. Given VOIPo's server uptime versus your outages (especially measured against others lack of downtime), it's safe to say with some authority that the problem is unlikely to be on their end. I am encouraged to hear that they have still be generous and helpful when helping you troubleshoot in the past, however.
Your troubles could be linked to noisy lines, or flaky router firmware, improperly managed port handling, packet priority. A lot can go wrong, unfortunately, when VoIP does go wrong. As such, it's hard to give you useful information without greater details about everything in your network and the quality of your ISP. Some folks have had great success in improving VoIP service stability just swapping the stock firmware from their router to OpenWRT or DD-WRT if they own a model with a supported build, or swapping out routers entirely for another make/model that has better VoIP packet management. Some router firmware can even degrade VoIP quality and stability by turning on packet management and enabling VoIP priority, ironically enough. Others have found relief in scheduling weekly router restarts in the firmware, too. Sometimes, there can be problems with the ATA as well, but those problems are mostly linked to planned obsolescence by the manufacturer and the power problems caused by failing capacitors on the circuitboard or the power supply after time. If this is a problem that you've seen from the get-go with VOIPo, and they shipped you a new ATA device, then this would be an unlikely potential cause to focus on so early.
As for alternatives, let it be known that Ting GSM and Consumer Cellular using their #2 network SIM cards (read T-Mobile - most store locations with their SIM cards in stock carry the AT&T version only) both offer and provide UMA WiFi calling now. To get this service to work, however, you need to have a SIM card that supports it, the service turned on at their end, and you need a T-Mobile branded smartphone that has the necessary support and firmware baked into it. This may open up a few options for you regarding mobile service, however. The cheapest handsets that'll get you there if you don't give much of a fig about stuff beyond calling and texting will be the Microsoft Lumia 435. T-Mobile branded and still carrier locked, used in good shape, you should be able to get one for under $20. I use a 435 myself on Ting, and the UMA WiFi calling feature is silky if you leave the preferences set to prioritizing mobile network over WiFi. (As with all things in life though, YMMV.) Billing on usage remains the same no matter how the call and text is placed, but there you have it. It probably won't fix the underlying network stability issues causing you problems in the first place, however.
