Openreach will shortly (if they aren't already) be using 2 suppliers for FTTC. They will match the modem to the chipset in the DSLAM. There are no guarantees a modem from another vendor will work. We tried a D-Link VDSL2 router on a line and their tech bods couldn't get it to connect (think it was in sync but wouldn't auth). The Thomson TG789vn worked but took some fiddling. It was a line that had a long standing fault and Openreach knew we'd replaced their modem without us telling them and were arranging to put a packet sniffer on the line to find out what we were doing.

Had we sent an engineer out with the Thomson connected to the line they would've refused to do anything and just charge for the visit.

I can't really see how we can stop people from using their own VDSL routers and replacing the BT modem other than saying you shouldn't do it and if you do you it may not work and you potentially won't be able to get support from Openreach in the event of a fault (or will have to pay).

I gathered the fact u still needed the OR modem, so I wold assume a "self install" would be like the old BTOpenworld ADSL when self install you had a stingray/frog modem being sent to you with a filter before activation date. I would assume similar with FTTC with the OR modem with either a soap on rope filter or faceplace being dispatched to you. You would then be given a date when the OR Engineer would go out to cab and make the switch. You would just plug in the equipment on that date.

But since this is about a CONTRACT between a service provider and a customer, ignorance IS an excuse when it comes to arbitrary T&Cs by an upstream party - I have yet to see ANY customer-facing terms and conditions by ANY service provider (BT or otherwise) that covers this. The closest are the phone line T&Cs by BT, and they merely state that anything before the master socket is not to be tampered with.

Nothing about VDSL modems anywhere - but I stand to be corrected. It would be interesting to challenge this if OR send out an engineer and then decides to ask for money. And in fact, it would be the third-party provider having to ask for that back, wouldn't it?

Considering VDSL2 should be a standard, the modem shouldn't make a difference - but I know it sometimes does in practice (see the U-R2 "standard" for ADSL and others). Still, I can't help but think that this sounds more like the VLAN problem with PPPoE than anything else... VLAN 101 does not seem to be widely publicised, maybe to deter people from using other modems or for future functionality (VOIP or BT Vision come to mind)...