Cisco 2600 Series Appendix page 11

Caller ID on Cisco 2600 and 3600 Series Routers and Cisco MC3810 Multiservice Concentrators
Command
Step 4
Router(config-voiceport)# caller-id alerting
dsp-pre-alloc
Step 5
Router(config-voiceport)# caller-id alerting pre-ring
Caller ID Configuration Task List
Purpose
(FXO ports, only when caller-ID alerting
line-reversal is required) Configure this command
on the FXO port when the incoming subscriber
telephone line uses line polarity reversal to signal
the start of caller-ID information transmission.
The Cisco FXO interface cannot detect
line-reversal alerting in the on-hook state. For this
reason, DSPs must be pre-allocated to serve the
Type 1 caller ID information when it arrives.
Preallocating the DSPs enables the DSP to
continuously monitor for the arrival of caller-ID
information.
(FXS ports only) Configure this setting only when
the attached telephone device requires the pre-ring
(immediate ring) method to signal the start of a
caller ID information. The command activates a
250-ms pre-ring.
Cisco IOS Voice, Video, and Fax Configuration Guide
VC-821