I discuss this a bit in footnote 1. Thousands of things were built on frames before computers, of course. However, IBM both built computers from semi-portable cabinets built around metal frames and also referred to the computer's units as "frames", including the "main frame".

Telephone systems used main distribution frames, but these are unrelated. First, they look nothing like a computer mainframe, being a large rack with cable connections. Second, there isn't a path from the telephony systems to the computer mainframes; if the first mainframes were developed at Bell Labs, for instance, it would be plausible.

As for Colossus, it was built on 90-inch racks, called the J rack, K rack, S rack, C rack, and so forth; see https://www.codesandciphers.org.uk/lorenz/colossus.htm