What exactly does the notation “s-BUSY” and “s-NOANSWER” mean? Is it a legal notation, or is it just naming the exten similar to, for example, “123”?
I understand that “s” is used as the default entry point in a given context for incoming calls that don’t have anything in the user part of the INVITE header or the “TO” header, so they wouldn’t be processed by this context at all without the use of “s.” But what does the combination “s-BUSY” or “s-NOANSWER” signify?
It’s an extension, just like any other. Their relevance or meaning is only within the scope of the rest of the dialplan. For example the first line checks “AVAILSTATUS” and jumps to the s-NOANSWER extension based on that.
It is not special. If it’s not referenced it wouldn’t be used, however something like s-${DIALSTATUS} where DIALSTATUS could contain BUSY would allow it to be referenced.