It has to do with the codes of signalling in the various protocols such as SS7, h323 and SIP. Taking SIP as an example code 180 is call progress connecting and

It has to do with the codes of signalling in the various protocols such as SS7, h323 and SIP. Taking SIP as an example code 180 is call progress connecting and a signal is generated that sounds like ringing while code 183 allows you to actually hear the sound being generated on the other end, which is how they create the custom ring tones or when you will hear the international ring tone. While 183 is the actual signal the 180 code is far more efficient and is what is used by the larger carriers.