There could be a couple of reasons for this;
1. I have seen this as a glitch in the software and resetting the unit can clear it.
2. Your provider is sending you too many calls and there are not enough DSPs available to handle the calls. This part is a little tricky. The A800 has 12 DSP channels, but only 8 are available for voice. The other 4 are extra and are used for dialing digits, and listening for answer supervision indicators. Here is where it gets a little tricky. If you are using the Tenor to recieve calls from IP and send to the PSTN, then when the call comes in from IP, you need 1 DSP channel for the call and then you need a 2nd dsp to dial the digits/answer supervision. Once the dialing is done and the call is answered the 2nd dsp channel is free for another call. So if you were to receive 4 calls at the same time, you will use 8 dsp channels till the calls are connected. 6 calls at the same time and you use all 12 dsps and the system will not be able to accept another call till a dsp is free.
check these out.