RFC 2833 specifies the use of RTP events to convey DTMF signals. This is a better way of sending DTMF signal over the IP network. However, the nature of the IP network is that it may from time to time experience network impairment such as delay and packet loss. So it is possible that the missing packets are caused by the network impairment. Users are suggested to perform the followings:-
1. Check the IP network using during the test. Ensure there is no bottleneck in it.
2. Perform a capture of the packet traces. Check the sequence number of the RTP stream. if there are missing sequence numbers, there are dropped packets during the course of traversing the network.
3. Make sure that you are not overloading the Abacus. In general, if you are having more than 1200 simultaneous call in one single ICG card, it is likely some packets are dropped before sending out to the network.