For anyone in the UK (although membership is open to anyone in the world), there is the Telecommunications Heritage Group, see
here. It's not a forum as such, but does have a lot of useful information.
If anyone has any old telephones they want to actually use to talk to people with, there is also
CNET - the Collectors Network, where people from all over the world connect anything from a single old telephone to full-blown telephone exchanges up, so they can be used to communicate with each other over the internet, for free. You might find me in the list of users if you look closely enough, if you get connected and want to, feel free to give me a call, I still have a couple of phones connected to the system, including
this one - notice anything odd about it..? Probably not unless you're as old as I am
I used to collect phones before I moved house, mainly old GPO stuff from the 1960's and 70's, but unfortunately I had to choose between phones and lamps when I moved, as there isn't room here for both collections
Lamps won, as you'll gather