VOIP not resetting after power outage

[Dhog2410]

We have recently moved to Vodafone Full Fibre and have successfully "Ported" the old landline number to VOIP.
We are currently having renovation works and the power has had to be turned off on 3 occasions. NB:We knew beforehand the phone would not be available while the poser was off.
The first time the power came back on, instead of a dial tone on the telephone handset, there was a high pitched whistle. If we made or received a call we could hear the other end but they couldn't hear us.
We called Customer Support who said "Oh I know what that is, I'll fix it from our end" and she did.
The second time the powere went out and came back on there was no problem.
The third time again the high pitched whistle so we swithched off JUST the ONT and router. When they came back on a few minutes later, the phone was again working normally.
My question is "Why is there a high-pitched whistle, rather than the system just rebooting normally and will rebooting the ONT and router always fix the problem"? 
We are also wondering if we should get a Battery Backup Unit and will this fix the problem?

[Beth]

Hi @Dhog2410! We're only able to supply a battery back up unit for accessibility needs, if you fall under this umbrella, it should stop any drop outs with your landline. Rebooting the ONT should fix any whistling issues as it comes from a misalignment with the digital line connection, however, hopefully you won't need to do this again in the future, unless there is another power outage 🙂

[Dhog2410]

Thanks Beth

From what I've read, although we have both have had heart attacks and are on the power Vulnerable persons list,  I don't think we would qualify for the FREE BBU as we have adaquate Mobile coverage.
The BBUs I've seen on line were under £100 and we are happy to purchase one but I haven't known what  BBU to get.

[Beth · Accepted solution]

@Dhog2410 I'd recommend speaking to our Wi-Fi Xperts team on 191 from a Vodafone phone, 03333 040191 from any other UK phone, or through your Vodafone Broadband app just to be sure that you don't qualify, as we'll be able to get this sent out for free if you are.

We wouldn't be able to recommend any specific brands, however, as long as you can see in their specifications that they are compatible with VoIP, any of those should be suitable for you 🙂