trixbox getting a busy tone on to ot 3 fxo modules

Hi, i'm using trixbox 2.4 it has 4 fxo modules but only 3 are being used ocasionally when people call in they get a busy tone for the entire day. say these are the telephone number 2222221 2222222 2222223 when i tested it with my cel and i called 2222221 and 22222222 i got a busy tone it normally goes to the irv but i does not when i call 2222223 it goes through. I swapped around the working 223 with the not working 221 and 221 work but not 2223 or 2222 this happens from time to time. but most of the time it works well. The number the customers normally dial is 2221 and it automatically sends them to the irv. if another custer dails 2221 when that person is on listening to the irv options they normally get the irv also so i know hat the lines are auto rolling. What would be the cause the occasional busy tone on two of the lines. and How do i fix it. One more thing when i reset the system usually everthing if okay.