Yes, but you have to have the SIP capability and analogue phone ports in your router. This pushes the price way up there (looking at one of the higher end Draytek models for example). The voice is simply a SIP endpoint, nothing fancy tbh - but most consumer routers don't include the ATA (Analogue Telephone Adapter) functionality.
IFAIK, the HG659 certainly has the "technical" capability to act as your single router - but you may struggle to make it work that way, as the firmware for this (and other similar devices) is controlled by the ISP and you can't get "generic" ones - so you may find the box is hard coded (so to speak) to only connect to Vodafone SIP servers (disclaimer I've not worked directly with this device so don't know)
That said, stick a decent router directly on to the ONT, and have the old tripleplay cabled to it and next to your phone (or bridged into your house jackpoints) and it's not too major a problem. The tripleplay does OK at the phone job.
You could also consider ditching the analogue phone altogether and get a SIP phone as your primary "fixed" line. Remember your phone doesn't work in a powercut any more anyway - so make sure you have a good reason to stick with the traditional technology
That is what I had expected about the HG659 - it certainly has all the hardware including ports; I will see if it can be configured for Slingshot VOIP. I'm not keen to ditch the phone altogether because we have quite a few around the house and I like the way the UFB router provides connectivity to the existing phone wiring. Cellphones are my reversionary devices for power cuts
So, it looks like the approach most likely to succeed is:
- configure HG659 for SS UFB connection
- configure tripleplay device to do just SIP and plug its WAN port into a HG659 LAN port
- plug existing phone wiring into phone port of tripleplay (this is how it is currently configured anyway)