Indeed the OP could use a VOIP provider such as Vonage over FTTP, as you can over any broadband connection. However the VOIP service would be dependent on the FTTP connection itself so wouldn't be much use in the event of a power cut - whereas a FVA service would continue to work using the ONT backup battery.
Nothing a cheap, small, UPS doesn't solve.
Also a VOIP service can work out pretty expensive if you dial a lot of 0845 numbers whereas the likes of BT can offer unlimited calls to them in their anytime call plans.
The OP will have to do the sums and make sure that the anytime plans make sense. Given the significantly reduced call costs of most VOIP providers (and no expensive call set up charges) it is likely that the packages aren't worth it...
How much do BT Retail charge for FVA by the way?
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