Also, a call that is closed due to PAP/CHAP authentication failure has a cause code of 16 because the call worked fine at the ISDN level, and was closed due to an indication from the PPP layer. In this case, if the local router decided that the PAP/CHAP username/ password it received were invalid, then the letter preceding the cause code will be U; if the remote router rejected the username/password sent to it, the letter proceeding the cause code will be N.
z17 - User busy
This is just like getting the engaged signal on a voice call. All you can do is wait for a while and try again later.
z18 - No user responding
There seems to be a device at the far end which has a LAPD connection to the switch but it is not responding to the call setup messages. This may indicate that the device at the far end has been set up to only make outgoing calls, or that it has a software or hardware fault that has put it in a state whereby it is capable of maintaining a LAPD connection to the switch, but can't actually responding to call setup requests.
z27 - Destination out of order
You are calling a valid number, but the switch does not have a valid LAPD connection on the channel corresponding to the number you are calling. This may mean that the device at the other end has not been properly installed yet, or is switched off. Or it may indicate a fault in the wiring between the switch and the site where that call is terminated.
z1 - Invalid number
The number you are calling is not assigned to any connection. Check the number you are calling.
Note: This cause code occurs if your router is connected to the centrex service, and you are calling a number outside of the centrex extensions, but have not preceded it with the prefix for an 'outside line'.
z34, 38, 44, 42 - ISDN network congestion or fault
If you receive these cause codes regularly, you should contact your ISDN network provider.
z21 - Call rejected
This is probably the most commonly seen cause code on failed ISDN connections. It almost always indicates a mistake in router configuration. This cause code occurs when the call setup packet is successfully delivered to the router at the other end of the connection, but the router refuses the call because it is not configured to accept the call. There can be are a number of configuration errors that can result in a call being refused. Common ones to look out for are:
z No ISDN call definitions have yet been created on the receiving router.
z The ISDN call definitions configured on the receiving router are set up to search for particular strings in the subaddress, CLI or
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