What can't be a telephone extention number, is it 0000, or something else?

I don’t think there is a technological reason for this. I have seen regular phone numbers end in -0000.

A long time ago during the days of dial phones, 0000 would take the longest to dial – your finger had to move around the circle fully 4 times on a dial phone, and with pulse dialing that was 40 clicks. (pre-touch-tone days).

I imagine that the number 0 is reserved for Operator in almost every switchboard, so having an extension that started with a 0 would just confuse the switchboard software. It wouldn’t know how to route the call.

Rules like this are governed by the North American Numbering Plan.

@elbanditoroso and @gorillapaws I’m asking about extension number, not telephone number. I have known lots of telephone numbers that end in 0000, but never an extention number that starts with 0, and for sure never all zeros.

@flo Extension numbers are governed by the system in each individual place. Someone could fabricate a system and install it at your house that would only respond to extensions 0, 00, 000, or 000000000000000

In other words, there are no hard rules there, and any patterns you’ve noticed are simply because people decided to do things a certain way.

@gorillapaws That sounds sound. Please post the link anyway.