Just a tale that shows how bundled mobile and broadband+VOIP could still fall over.
I don't have a landline. Just Three, for mobile and for a Home Broadband Three router.
Three went down completely for a while, leaving me with nothing.I was expecting an important phone call from an NHS hospital in the 1pm-5pm time-slot. At noon the service was still down.
Luckily I have my previous phone stuffed away in case a complete fail like this happens. With a £9pm 5GB PAYG, that does accumulate up to 12GB or so. Tried to add it to my NHS account, then call the hospital to alert them to the alternative number,
Neither the App nor the website will let you have a "landline" or second mobile. Luckily Three came back up about 12:30, and the incoming call at 2pm.
End of my tale, and I can implement my plan for most purposes by pretending my old mobile is a landline number. (Problem - specifying the default as simply reversing the roles on most sites could make things worse).
What it shows however is that before long a lot of people will be forced to rely on a bundled mobile if broadband goes down, but if the bundled mobile service is also off they could well be stuffed.
Connections: OnePlus 8 Pro on Three 4+ (LTE)/5G and at home Three Mobile, with (Three)ZTE MF286D router giving about 113/20Mbps.