Secondary dialtone

In telecommunication, the term second dialtone has the following meanings:

1. Dialtone presented to the call originator after an access code has been dialed for access to a second, outside, telecommunications system or service.
2. Dialtone returned to the call originator after she/he has dialed an access number and has reached a switch providing access to modem, to a fax machine, to another telephone, etc.

Source: from Federal Standard 1037C