Hotline/Warmline, also known as Private Line Automatic Ringdown (PLAR), is a line used for priority telephone service. When using with a SIP phone you must make sure the phone supports this feature. As of this writing I know that Polycom SIP phones support sending a signal to Asterisk upon the offhook event.
Asterisk DAHDI supports this feature on FXS extensions using the immediate=yes and the call will be passed to s extension in the related context. In addition iax protocol also supports this feature.
I have also tested this with Cisco ATA186. Here is the way to implement PLAR with Asterisk for this particular ATA:
If the Hotline feature is configured, the Cisco ATA immediately dials a pre-configured number as soon as the handset goes off hook. If the Warmline feature is configured, the Cisco ATA dials a pre-configured number if no digits were entered before the specified timer value expired when the handset went off hook. This is to be set in the ATA GUI > Service Parameters > DialPlan.
where d is a delay-in-seconds parameter 0-9,a-z (to support 0 to 35 seconds delay), and nnnn is the variable-length phone number to call when no digits are entered for d seconds after the handset goes off hook.
Example 1: H05551212 (Hotline configuration; the Cisco ATA immediately dials 555-1212 when the handset goes off hook.) Example 2: H55551212 (Warmline configuration; the Cisco ATA waits for five seconds and dials 555-1212 if no digits were entered when the handset went off hook.)