Music on hold interruption a telephone conversation


We have an asterisk PBX (ver. installed with TrixBox (kernel version: 2.6.18-53.14.el5). We are also using hylafax for receiving faxes and is now functioning normally. would greatly appreciate if you help me about a problem on which I am finding no information in either this forum or other forums or google in general.
The problem is that when a user is in the midst of a telephone conversation the music on hold suddenly jumps out and interrupting the conversation. To spend a few seconds (2 or 4) for music and then restore the communication. Sometimes a few beeps sound background in the middle of the conversation which I assume correspond to error beeps emitted by the asterisk but will not know what is wrong.
I have checked the logs of asterisk you offered me some clue as much but I found that sometimes overlaps the time of the interruption hylafax cyclic testing but does not always match so I do not know if the problem is that interrupts the hylafax conversation is in progress. With the routine of checking the implementation, I mean "restricting registration for peerâ€