One of the differences between using VoIP for phone calls and using analogue wired phone lines is that that a VoIP service is dependant on a number of other things working, take for example the diagram:

as you can see there is quite a lot more involved in making a call over VoIP compared to over the traditional analogue phone

VoIP needs local power for the phone
VoIP needs local power for the router and FTTP ONT
VoIP is reliant on the internet connection working
Analogue phone doesn't require power
Analogue phone connects directly via a cable all the way through to the local BT exchange

Considerations

As you can see there is a lot more to go wrong. The main things to consider when using VoIP is:

What happens if I lose power
What happens if my internet connection has a fault

This is especially important to consider when you start to think about access to emergency services.

What happens if I lose power

Losing power will mean your internet connection goes off and your VoIP phone has no power and no connection to the internet. You'll be unable to make calls.

Solutions:

Use a UPS - an uninterruptible power supply. - typically a heavy box that contains a battery that is kept charged up and will switch to battery power in the event of losing power.
Use a mobile phone - Keep a mobile phone close to you and on charge - use this to make phone calls if you lose power.

What happens if my internet connection has a fault

A fault on your internet connection will probably affect your VoIP service. You'll be unable to make calls.

Solutions:

Use a mobile phone - Keep a mobile phone close to you and on charge - use this to make phone calls if you lose power.