I’ve recently continued having problems with my Wife’s VPN, using PPTP VPN (the one you use by default in Windows). Rather than timing out during authentication, which was what I was getting when the router seemed to be blocking it, these days it seems to be rejecting it immediately. I’ve tried enabling VPN passthrough on my DD-WRT (ex-Virgin) router, and even connecting directly to the cable modem with no router, but I still get the same problem.
Then, occasionally, it works.
However, the interesting thing, is that the IT Admin for her work (a large UK university) says he’s had 4-5 other people also having this problem – all Virgin customers.
I doubt they’re deliberately blocking it – but perhaps something in their configuration isn’t letting PPTP through?