SIP is based on request/response transactions. Each transaction consists of a SIP request and at least one response.
“486 Busy Here”
When the carrier sends in a request to start a new call, and there are no more licenses to fill that request, the system responds to the INVITE with a “486 Busy Here”.
This can be configured to return any other SIP response code by setting this value in the HmpElementsServer.exe.config file:
<setting name="ServerBusyResponse" serializeAs="String">
<value>0</value>
</setting>
When this is set to zero, then the default “486 Busy Here” is sent.
“503 Service Unavailable”
When this is set to some other code, like 503, then a “503 Service Unavailable” is returned.
Valid SIP Codes
Valid SIP codes can be seen here:
https://en.wikipedia.org/wiki/List_of_SIP_response_codes#4xx.E2.80.94Client_Failure_Responses
Your Carrier’s Response
What your carrier decides when it receives one of these codes is defined by your carrier. Some carriers will play a busy signal, others will not. Some will play a message that the number cannot be reached. Some carriers can be defined to roll the call over to a backup IP address; it all depends on your carrier.
OPTIONS Requests
If your carrier supports OPTIONS requests you can give the carrier a “heads-up” about your server’s status. Please see this article on our wiki: SIP Options in HMP Elements