234 CHAPTER 10: SIP-M ODE O PERATIONS■ Virtual Tie Lines are not available on a system running in SIP mode.However, you can achieve the same result, connecting differentsystems, by configuring each system that is running SIP mode as atrusted SIP interface. A trusted SIP interface can include SIP proxies,SIP applications, SIP gateways, and any other third-party SIP device,including 3Com VCX IP Telephony systems. Telephone communicationbetween the NBX and VCX systems supports Make Call, Receive Call,and Call Hold. Each NBX system can use the same IP MessagingModule.■ A Trusted SIP Interface connection between an NBX system and a3Com VCX telephone system can share the same IP MessagingModule. However, the automatic mailbox creation process can workfor only one type of system. Typically, you use automatic mailboxcreation for the enterprise-class VCX system and manual mailboxcreation for the NBX system or systems.■ Paging is supported on 3Com phones (except the 3Com 3108Wireless Telephone) with SIP enabled. Generic SIP phones and the3108 Wireless Telephone can neither initiate nor receive pages.■ There are restrictions on the Automatic Call Distribution (ACD) featureon a SIP mode system. See “SIP Mode and ACD” on page 239.■ Directory services are not supported on generic SIP phones and 3Com3108 Wireless Telephones.■ SIP mode systems support Attendant Console operations. However,only call status, not line status, of SIP endpoints is available for SIPtelephones.■ A TAPI application is only able to monitor generic SIP phones and the3Com 3108 Wireless telephone.A SIP mode system supports E-911 functionality. However, for 911 calls,you must manually configure generic SIP telephones and the 3Com 3108Wireless Telephone to use the alternate SIP gateway address if the CallProcessor is not available. For generic SIP telephones, this behavior isspecific to the telephone, and it is the responsibility of telephone toprovide this functionality.Device SupportDetailsThere are important distinctions to keep in mind when you consider howdevices connected to a SIP mode system behave:■ SIP-only telephones such the 3Com 3108 Wireless Telephone andgeneric, third-party SIP telephones can use many of the standardtelephony features through feature codes.
