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Dial the main number, wait for the greeting or dial tone, then dial the extension. On a fax machine you insert pauses, written as commas, between the main number and the extension so the second set of digits is sent after the call connects. Online fax services usually give you a separate field for the extension.
An extension is not part of the phone number the way an area code is. It is instructions for the company's phone system after your call arrives: "now connect me to desk 412." The fax machine has to deliver those digits at the right moment, which is what the pauses are for.
When you program a fax machine's dial string, each comma tells it to wait about two seconds before sending the next digits. So a dial string like 4165550100,,,412 dials the main number, waits roughly six seconds for the greeting to start, then sends 412. If the greeting is long, add more commas. If the digits go through too early, they are swallowed by the greeting and the call goes nowhere.
| Symbol | Meaning | When to use it |
|---|---|---|
| , (comma) | Pause about two seconds | Between the main number and the extension; stack several for long greetings |
| ; (semicolon) | Wait for the user to continue (on some machines) | Rare on fax machines; more of a phone-dialer feature |
| w or W | Wait for a dial tone before continuing | When the system plays a second dial tone after you enter a menu choice |
| p or P | Pause (same as comma on many machines) | Alternative to commas on machines that use P instead |
Then you are dealing with a menu, not a simple extension, and the timing gets fussier. A typical flow: dial the main number, wait through "thank you for calling," press the digit for the department, wait again, then enter the extension. Each wait needs its own commas, and the total dial string can get long: 4165550100,,,,2,,,412 is not unusual.
The practical trick: call the number from your own phone first and count how long each stage takes. Then build the fax dial string to match what you heard. If the attendant's menu changes seasonally ("press 3 for our holiday hours"), your programmed string will break silently, and the fax will fail with no useful error. Re-test after any menu change.
No. If someone gives you "fax: 416-555-0100 ext. 412," the fax number is 416-555-0100 and 412 is routing instructions. Write down both parts separately. Online fax services handle this cleanly: you put the main number in the number field and the extension in the extension field, and the service times the digits for you.
Work through the failure points in order. First, confirm the extension with a voice call: if a human cannot reach the fax machine at that extension either, the problem is the routing, not your fax. Second, lengthen the pauses: most extension-dialing failures are timing failures, and adding two more commas fixes more of them than any other single change. Third, check whether the company's system expects the extension before or after a menu selection; some systems want the full extension immediately, others want a department digit first.
And one honest simplification: if you are faxing through LetterAgent (currently in testing), you do not build dial strings at all. Tell your AI assistant the main fax number and the extension, and the sending side handles the dialing. US and Canadian fax numbers only.
Ready to fax it? Fax with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is sent.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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