Hi there,
Interestingly, I've just had a similar problem to you. I've just moved house and placed a new order on my line. I kept my netgear router plugged into the phone socket so I could see if it was activated early. Activation date passes and I'm still not seeing any activity, so I locate my test socket and manage to sync briefly until I reboot the router. Doesn't sync again.
BT Engineer comes out and it works fine with his kit. So, my router had decided to die
Tried with another router and all is now fine, but quite annoying!
Can you please confirm that you're still not able to sync with the exchange now even if you are connected with the test socket? If you could try using another modem, it would be really handy, just to make sure that your Voyager hasn't blown up like mine did!