I found a great working telephone exchange exhibit at the "Internal Fire Museum of Power" https://www.internalfire.com/. It's mostly a museum of diesel engins, but in a separate building at the back they have a telephone exhibit.
They had several old telephone exchanges connected together so you could place calls between them and also between rooms elsewhere in the museum. The oldest exchange was a manual plug board type, the newest was an Asterisk PBX running on a Raspberry Pi which had an IP desk phone connected to it. It was mostly all connected together, although I didn't get very long to play with it, so I'm not completely sure.
Reminds me of https://this-museum-is-not-obsolete.com/ - which is more about using old technology (art rather than preservation, but also more of an emphasis on being able to see the step-by-step relays operate while you're dialing.)
We used to have an absolutely fantastic museum called 'NINT' in Amsterdam, it had 10's of live exhibits like that and none of it was supervised. Then it got moved and half the collection was lost, then it was moved again and the lost the other half and now it lives on as NEMO, next to central station. It's still a nice day trip for younger kids but the hard technology angle has been almost entirely erased. Not that any kid today would understand what a rotary phone switchboard does anyway...
They had several old telephone exchanges connected together so you could place calls between them and also between rooms elsewhere in the museum. The oldest exchange was a manual plug board type, the newest was an Asterisk PBX running on a Raspberry Pi which had an IP desk phone connected to it. It was mostly all connected together, although I didn't get very long to play with it, so I'm not completely sure.