====== SIP SRST ======
  * Provides backup SIP proxy-server functions to SIP phones
  * Supports up to 480 SIP phones
  * Provides backup to an external SIP proxy server by providing basic registrar and B2BUA services
  * Supports the following call combinations
    * SIP phone to SIP phone
    * SIP phone to PSTN
    * SIP phone to SCCP phone
    * SIP phone to WAN VoIP using SIP