The problem is the voip otherwise you could just get a fritzbox 7520 or 7530 off ebay. Yes the fritzbox supports voip but openwrt doesn't support the voip chip on the fritzbox (there's no device you can buy that has openwrt+vdsl2+voip support as far as I know there is only openwrt+vdsl2)
So basically you should probably just keep the modem/router that's there, disable the wireless radios on it and get a new wireless router that lets you put openwrt and from there you can configure it to work with nordvpn but that wil take a bit of configuring to work, it's not just click click there you go but it's totally possible.
Maybe there is the option of getting a fritzbox 7520 or 7530 and then voip gear but then like moeller0 says exactly how to configure that to get it to work you'd need to find someone with exact experience, get passwords etc
I don't know your circumstances @nedp but have you considered if you really need BT's voip service? I also wondered are you absolutely sure that BT has put you onto digital voice? Most FTTC would still be via a landline completely separate from the broadband connection. You have to specifically ask for it I'm pretty sure.
One idea that is popular in the UK is once your contract with BT is up (or close to being up) just port the number to a very reputable provider in the UK called Andrew & Arnold (A&A). Then you can have your number on a VoIP handset with the SIP details given to you by A&A and have any router you want. Then just resign up with BT taking advantage of all the cashback and welcome offers (£70 prepaid mastercard etc) via sites such as topcashback.
Not quite, things like https://openwrt.org/toh/arcadyan/vgv7510kw22 can do that (VoIP towards the ISP, two analogue phone ports and/or SIP to your internal phones), via chan_lantiq and asterisk (which is possible, but rather hard to configure). The bigger problem is finding a device that does have FXS ports (DECT is never supported), and sufficient amounts of flash/ RAM (as you see in my example, 16/64, very borderline - especially with asterisk onboard) and decent wireless (again, 2.4-GHz-only RaLink wireless in my example).
In theory, it's possible.
In practice it doesn't make sense, as the compromises are just not good - especially as lantiq vr9 and 802.11ac wireless options are getting quite long in the tooth. If you want a good solution, you will inevitably end up with a 3-device-setup, modem && wireless router (OpenWrt) && VoIP-systems (AVM's Fritz!Boxes with their OEM firmware can cover this); the AVM Fritz!Box 7520/7530 (first generation!) is one of the very few options that could combine modem && router into one device running OpenWrt (phone features still need to be handled differently).