Far easier to repair (i.e., to source parts) for vacuum tube audio than modern audio. E.g., the 2A3 power triode has been in continuous production since it was introduced in the early 1930s. Similarly the 6SN7, 12AX7, and other common audio/hifi tubes have never gone out of production. EDIT: people say it's because of the musical instrument amp market, but it's more subtle than that. I don't think you'll
ever find a guitar or harp amp rockin' a 2A3 direct-heated triode.
"We" tend to eschew obsolescence.
As to a tube tester... I mean... I have one, but that doesn't help much. They're of somewhat limited usefulness, anyway. The best test of a tube is
in situ (in the circuit for which you wish to use it).
Could well
not be a tube, though. They certainly can fail catastrophically (e.g., power output tubes and HV rectifiers) but it's not terribly common (at least with small signal tubes). Rectifiers in that intercom are probably solid state. Power tube is probably something like a 6AQ5 (single ended or two for push-pull). Danged rugged and cheap tubes even today.
I presume the intercom is a NuTone. They were the
sine qua non for house intercoms.
What does it do, or not do?
Feel free to send me a PM and we can troubleshoot by wire.

PS if you have a
chassis number, send it to me and I'll see if it's in Sams. If so -- I probably have the schematic.