SS7 is signalling protocol used to route telephone calls (analog and digital/VoIP, no different) between CLEC's (telecom providers such as Bell, Verizon, AT&T, etc..) SS7 is an international standard that all telecom's around the world use. Generally, there is little modern encrption or authentication between telecom's on SS7.
Link to a good SANS institude article on SS7 security.
Generic SS7/Telco Cause Codes
Here is a listing of SS7 ITU Standardized related cause codes:
0 - In Progress: Call in progess, that is a good thing..
1 - Unassigned Number: This cause code means that called party (the number you are dialing), cannot be reached becaue even though the number is a valid number, it has not been assigned to anyone...
2 - No Route to Transit Network: This cause code means that the equipment sending you this cause code has received a request to router the call through a particular network (transit network), that doesn't serve the desination required. This cause code is supported on a network by network basis..
3 - No Route to Desination: Here the called party cannot be reached the network which you are using to get to the called party does not serve the destination desired.. Again, this is supposed on a network by network basis..
4 - Send Special Information Tone: This cause code means that the called party cannot be reached because of long term problems and that the special information tone should be returned to the calling party..
5 - Misdialed trunk prefix:
6 - Channel Unacceptable: This cause code means that the channel identified is not acceptable for use for this call.
7 - Call Awarded and Being Delivered in an Established Channel
16 - Normal Call Clearing: This cause code means that the call is being cleared because one of the parties involved hung up.
17 - User Busy: This cause code means that the called party is busy with another call, and thus can't accept your call.
18 - No User Responce: This cause means that the called party did not respond to a call establishment message with either an alerting or connect message within an acceptable amount of time.
19 - No Answer (user has been alerted): This cause code means that the called party user has been alerted but decided to not answer/respond to your connect request within an acceptable amount of time..
21 - Call Rejected: This cause code is pretty self explanitory.. The called party, or their equipment, does not wish to accept your call, and has rejected you even though the called party is not busy on the phone. Depending on the network equipment, there is a chance that in the diagnostic field there might be additional information about the reason for rejection.
22 - Number Changed: This is an optional cause code, and means that the called party number is no longer assigned, but that a new number has been assigned which might be optionally included in the diagnostic field..
23 - Unallocated Destination Number: Pretty simple, the called party number you called isn't assigned to anyone..