The AT0 trunk is an analog trunk. The connection principle is as follows:
The AT0 trunk starts a timer whenever a user dials a digit. Assume that the called number is 123456789 and the minimum number length is 5.
1. The calling party picks up the phone and dials the called number. The AT0 trunk collects the first five digits (1, 2, 3, 4, and 5) and stores them locally.
2. When the first timer times out, the AT0 trunk sends the first five digits stored locally. The AT0 trunk sends only one digit at a time, and waits for the next timer to time out before sending the next digit.
3. When sending the first five digits, the AT0 trunk continues to collect the digits dialed by the calling party. After the first five digits are sent, the AT0 trunk continues to send the following digits one by one when each timer times out.
4. If all digits stored locally are sent but no digit is collected when the next timer times out, the AT0 trunk sends the connect message to the peer office.
5. If more digits are collected after the connect message is sent, the AT0 trunk packs these digits into audio for transmission. (Digits transmitted in audio will not be recorded into U1910 CDRs.)
The following figure shows the process.
According to the fault symptom, the possible cause is that the parameter settings for the AT0 trunk are not optimized.