This is the timestamp marking when the call was answered and billing began, formatted as YYYYMMDD:hhmmssms, year, month, day, hour, minute, second, and milliseconds, per the format the description spells out. Because this specifically marks the start of billing rather than call setup or dialing time, it's the field a billing reconciliation process anchors on when computing call duration and charges, and any clock drift on the DS1 device would directly skew billed durations. For example, the description's own sample value '20010506:172335100' represents May 6, 2001 at 5:23:35 PM and 100 milliseconds, the exact moment billing started for that call.