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I understand that someone's work phone might have an extension. What do you call the main number of that office, which would normally be answered by an operator or a computer voice system?

Would it be the "Main Line", the "Main Station", the "Root of Extension"? I would like to know the technical name as well as some more informal or slang terms, if there are any.

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What’s wrong with what you already have, “main number”? By the way, the normal way to call the main telephone is with another telephone. :) – tchrist Dec 13 '12 at 6:07
Operator -- not technical, not slang, but the one that anyone uses in the context. Connect to the Operator! even when it's a non-human device. Zero: get zero (connects to "server") – Kris Dec 13 '12 at 6:13
I would simply call this the phone number. "Here is my phone number; here is my extension." Or, "Jenny's phone number is 867-5309, her extension is 22." – J.R. Dec 13 '12 at 9:18
up vote 6 down vote accepted

From the days when large offices had their telephone lines connected through a mechanical switch, you would ask for a number that will connect you to the switchboard or the switch operator (or when it was not deemed to be sexist, the "switch girl").

As they became more technologically sophisticated, corporations might replace their switch with a private automatic branch exchange (PABX). In that case, their central number would connect to that and calls would be distributed to the EXT (extension) lines.

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+1 Switchboard for me – Henry Dec 13 '12 at 7:53

I hear main line a lot but main (phone) number is fine too. If you tell many people (of course not ELU users) that here is my switch(board) number you might leave them confused. Also google and other search engines are spitting out main phone number far better than switch(board) number.

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''Board number'' would be the word sounding more correct in my ears.

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Welcome to EL&U. We normally expect reasons or citations in an answer: otherwise it's just one person's opinion, which isn't very helpful. "Board number is what we use in (company/city)" would be better. – TimLymington Dec 13 '12 at 11:32
This depends on context, too. If I was talking to our comm guys about a new configuration of the company phone system, we might use "board number." However, if I was at a conference and ran out of business cards, I doubt I would ever say, "Let me give you my board number..." – J.R. Dec 13 '12 at 11:54

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