the only point I can agree with you here is the fault tolerance and it being a single point of failure.
With any home voip service your internet connection is the limiting factor, so you already have this QOS factor in place. Plus the mobile networks are more more jittery and have a higher latency then your home service could ever dish out.
If you were concerned about your home service you could configure asterisk to not act as your media server and instead would be used for signaling the media would be point to point from your mobile to "voipo"
If you were more concerned about the quality of your mobile provider you could configure asterisk to act as a media server and transcode to GSM/729/ILBC/ect and provide buffering between the two services.
If excellent voice quality, no jitter, and no latency were your goals then you wouldn't even consider 4g connectivity
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