With those requirements, you will have to go with (3) dedicated devices for modem, router and sip ATA.
The only OpenWrt supported SOC with VDSL support is lantiq VR9 (VRX268/ VRX288), which is fine up to profile 17a, 30a - however it doesn't support super-vectoring (VPlus/35b). The newer xRX3xx/ xRX5xx lantiq chipsets which could support it aren't covered by OpenWrt yet. Furthermore as a SOC, the lantiq xrx2xx chipset isn't the fastest (especially without proprietary hardware acceleration), which would be limiting towards the upper end of your expectations (if you want to go this route, the BT Home Hub 5 Type A is as good as it gets). So if your requirements are set in stone, you will have to use a dedicated (non-OpenWrt) VDSL modem.
There are plenty suggestions for highend routers, so I don't think that question needs to be repeated here.
In terms of SIP support, you can look at lantiq VR9 again, which has FXS support under OpenWrt via asterisk and chan_lantiq - or you could dabble into commercial offerings, native SIP phones, etc.
Yes, with somewhat adjusted expectations you can do all three tasks with OpenWrt supported devices - but performance and the available selection of supported devices won't be sufficient to do it in a single device (with one core lost to voice processing/ FXS support, effective WAN throughput on lantiq VR9 with OpenWrt is limited to roughly 55 MBit/s).
For practical reasons (performance, feature and security), using a dedicated NAS type device would also be a better choice, but highend mvebu or ipq806x might also do a decent job there.