Re: Yes but no but
Given the likes of VM are shutting off their POTS (I had to move over to a dongle on the modem last week*), and offer to supply an "emergency handset" if you're "vulnerable" I would very much have hoped that the system to have your mobile automatically work with any tower without any additoonal cost to you would have been in place already.
Related to this is the fact that the mobile companies are determined to get everyone onto expensive contracts, evne if they just want "PAYG"
A friend who has an old PAYG sim that he was very careful to keep in credit for his mobile he mainly uses for emergencies/taxis (he's disabled, on a limited budget and basically suck in the house) was furious that his phone provider apparently cancelled the sim without any warning to him, no message about "you need to make a call in the next week" or "you need to top up again in the next week to retain functionality" the first he knew was when he tried to buy something and didn't get the code SMS from his bank and someone not being able to contact him via his phone mentioning it in an email.
In his case he rang up and after a discussion and the call handler talking to a manager they reactivated the sim on the old number but without any credit.
I've since found the emergency phone I bought for my dad has done the same thing because he didn't use it for a couple of months, so the loss of physical landlines if there is a power cut means that unless you're on a contract phone there is a very good chance your occasional use mobile is going to be worthless when you need it most.
*The "Instructions" supplied made it seem like you needed to call them if you had a simple double adaptor for your phone.