Voice API Programming Guide — June 2005 69
Call Progress Analysis
Some telephone systems return a momentary drop in loop current when a connection has been
established (answer supervision). Loop current detection returns a connect when a transient loop
current drop is detected.
In some environments, including most PBXs, answer supervision is not provided. In these
environments, Loop current detection will not function. Check with your Central Office or PBX
supplier to see if answer supervision based on loop current changes is available.
In some cases, the application may receive one or more transient loop current drops before an
actual connection occurs. This is particularly true when dialing long-distance numbers, when the
call may be routed through several different switches. Any one of these switches may be capable of
generating a momentary drop in loop current.
To disable loop current detection, set DX_CAP ca_lcdly to -1.
Note: For applications that use loop current reversal to signal a disconnect, it is recommended that
DXBD_MINLCOFF be set to 2 to prevent Loop Current On and Loop Current Off from being
reported instead of Loop Current Reversal.

7.11.7.1 Loop Current Detection Parameters Affecting a Connect

To prevent detecting a connect prematurely or falsely due to a spurious loop current drop, you can
delay the start of loop current detection by using the parameter ca_lcdly.
Loop current detection returns a connect after detecting a loop current drop. To allow the person
who answered the phone to say “hello” before the application proceeds, you can delay the return of
the connect by using the parameter ca_lcdly1.
ca_lcdly
Loop Current Delay: the delay after dialing has been completed and before beginning Loop
Current Detection. To disable loop current detection, set to -1. Default: 400 (10 msec units).
ca_lcdly1
Loop Current Delay 1: the delay after loop current detection detects a transient drop in loop
current and before call progress analysis returns a connect to the application. Default: 10 (10
msec units).
If the ATDX_CONNTYPE( ) function returns CON_LPC, the connect was due to loop current
detection.
Note: When a connect is detected through positive voice detection or loop current detection, the
DX_CAP parameters ca_hedge, ca_ansrdgl, and ca_maxansr are ignored.
7.12 Media Tone Detection on Springware Boards
Media tone detection in call progress analysis is discussed in the following topics:
•Positive Voice Detection (PVD)
•Positive Answering Machine Detection (PAMD)