Replacing Plusnet router firmware with OpenWRT - old Thompson

[#1]

Hello, i'm looking to recycle an old spare router I have to run my  PBX phone over VPN - the Cisco Spa 921 I have is so old this technology is not built in. Is it possible to flash the router with Open WRT so I can do what I want with it as a go between - effectively i'd be connecting two Thompson's together as the current router I have to connect to the internet is another Thompson box.
Many thanks 🙂

[#2]

Also just a quick further question, are the old Thompson (Technicolor) routers Broadcom based as apparently there is an issue with there being no open source drivers for them and me potentially losing my ability to use wifi if I decide to simply flash one router and try and VPN the lot 🙂