be developed. One trend starting in the SP offering of SIP trunking services to call centers is the capa- bility to support many different methods to move calls in and out of call centers. Historically, trunks sold to call centers utilized either a "*8" transfer method, SS7 signal- ing for call transfer, or Two B channel transfers (TBCT). Each of these TDM-based call transfer mechanisms has strengths and weaknesses, as shown in Table 2-4. As customers adopt SIP trunks for call centers, the first method that was used was a repli- cation of the existing services. However, a trend toward a more SIP-specific method for transferring calls between call centers is developing. This trend is toward the use of Refer and variant of this signaling method to move calls with a high degree of confidence and with the capability to carry information from one call center to another. A second major trend is the support for teleworkers. As home-based teleworkers adopt the use of SIP trunking for their voice communications, the first option provided to them was to extend the WAN coverage of the enterprise to the teleworker's house. This required that a tunnel be created from the remote worker to the enterprise and that the enterprise use a centralized SIP trunk. Chapter 2: Trends in IP Telephony 29 Table 2-4 Strengths and Weaknesses of TDM-Based Call Transfer Mechanisms Transfer How Does It Work? Applicability to SIP Downside of Method Trunking Methodology When a transfer needs to be "*8" transfers can occur The use of a "*8" transfer initiated, an audio signal is across SIP Trunks. IP-PBX can be heard by the caller, sent from the end IP-PBX to connected to SIP trunks can is not guaranteed to work, the network in the audio play the same audio tone, and require that DTMF path. The network is listen- and this will continue to be digits be sent within the ing at all times to the audio detected in the network. audio stream. These of a call. When a "*8" is Cisco voice gateways and requirements limit the detected in the network, the Enterp