First things to do:
As for the other questions:
1) Will a broadband connect be needed. Yes in order for the phone service to work some form of broadband connection will be installed which may include a router and possible a ONT if in a Full fibre area.
2) What about powercut? Provide your communication provider with the situation and if they have no mobile phone or even mobile signal. The provider will then provide an appropriate solution and guidance. For example BT may offer a battery backup or hybrid phone.
3) What about installations? Again let your communication provider know about the individuals situation and if extra help is needed to set it up.
The answers to each of your points has been addressed, so if you have read each message you should already know , but to recap .
Yes , IP telephony will require a broadband modem ( not really a router but close enough )
The wired phones around the house will only continue to work if they integrated into the new way the phone service is delivered, however the current rollout to broadband customers can include a DV adapter that converts a wired phone into a DECT ( wireless ) one , but someone with a wired extension phone in every room will need to incorporate the wiring for them to continue to work .
The alarm is a question for the alarm company, alarm companies have been aware of the change for years , the chances are it will operate in exactly the same way as now, but the onus is for the user to check with their alarm provider.
In a power cut the service won’t work unless the modem/router is connected to a battery back up unit ( BBU ) or a uninterruptible power supply ( UPS ) , BT may supply a BBU for those that qualify, or the customer could buy one themselves if this is a concern but they are not deemed to be in a qualifying group
For all that , if your mother has a phone only service , then a migration to DV is likely to be later rather than sooner ( the last date December 2025 ) , there is also a product that may be introduced for phone only customers where the modem/router ( actually an ATA ) is housed in the exchange building ( the same one that the PSTN equipment has been switched off in ) , in which case there is no practical change at the customer end at all ,same phones , same wiring , no more affected by power outages than the current system, but it seems no BT phone only customers have actually been migrated to DV , or if they have they haven’t made their experience public , so for BT phone only customers there is likely still to be over 20 months still to go .
Some useful information here https://landlinesgo.digital/
if you have read every post then your questions have already been answered but someone will post answers again
is your mother registered here https://www.bt.com/help/here-for-you
Thank you to everyone who has been helpful in clarifying these points - much appreciated. If those answers were already in the thread, I suspect it needed background knowledge I don't yet have to have realised that. There's a lot of jargon around this.
If my mother does need some kind of "powered box" (apologies for using the wrong term) to connect her existing phones and sockets to the new service, do know what that looks like and how large it is? I need to work out.if.we.can somehow fasten it to the wall above the skirting board, or if we need to build some kind of cupboard or.shelving for it. She's not going to want it sat on the kitchen table near where the main BT socket is. Also. If it needs power, will the BT installation engineer be arranging that, or is the expectation that we spend our own money on an electrician to do it for her?
I don`t think you are going to get the answers to the questions regarding the location of the "interface" unit, or how its going to be powered, but I would not expect anyone from BT to visit, and I suspect the "interface" unit, whatever its going to be, will simply be delivered by post, and it will be up to the end user to plug it into a suitable mains supply.
First of all, as has already been said, it is not finalised what the solution for telephony only customers will be. Secondly, it will be well into next year before telephony only customers are moved to DV. She will be given plenty of notice of the transfer giving ample time to arrange any possible electrical work to be done.