help please - cannot grab an outside line

[soupmix]

Our current system is set up with all of our outside lines grouped to three buttons on the phone. A user must enter an account code to get an outside line. However, about 50% of the time,when attmepting to do so, the user does not get a dial tone, just silence. Does anyone know why this is happening and what I can do to make sure our employess are getting an outside line when they

 

[fonedude]

It sounds like 1 line in one of those groups is dead. If you can us the attendaant phone to monitor which light comes on, you can determine which line it is.