ed solution is chosen, MWI indi- cations are provided via SIP indications (as per RFC-3842) from the service provider sys- tem and are relayed to the enterprise endpoints. Be sure to test these scenarios for SIP trunking if this is your deployment model. Dual-tone multi-frequency (DTMF) interworking is often needed for voice mail as well. Even if all the communicating systems use SIP, there are various ways of relaying DTMF in SIP. DTMF interworking is discussed in more detail in Chapter 9, "Questions to Ask of a Service Provider Offering and an SBC Vendor." Transcoding When offnet access was provided by TDM access to the PSTN, codec choices were entirely within the control of the enterprise. Codecs were configured on IP endpoints, enterprise applications, and TDM PSTN gateways. Codec choices did not affect offnet calls in any way. With SIP trunking entering your network, this is no longer the case. Codec choices are now end-to-end on the IP segments and enterprise endpoint negotiated codecs with external endpoints and application controlled by other networks. Codecs offered by external endpoint might be against the bandwidth (call admission control) policies of your enterprise network, or your older endpoints might be incapable of supporting some of the newer codec choices, resulting in failed calls or inappropriate bandwidth use on your network. Sometimes SIP trunking is a cost-effective choice only when G.729 is the codec chosen (for bandwidth delivery reasons), especially for high volume contact centers operations. For all these reasons, it might be necessary to do transcoding at the border of your enter- prise network to change the codec to the appropriate one before these calls enter your network. Local Digital Signal Processing (DSP) can provide transcoding for a call that uses a high-bandwidth codec such as G.711 on one side and a low-bandwidth codec such as G.729 or Internet Low-Bitrate Codec (iLBC) on the other side. Transcoding is dis- cussed in more detail in Chapter 9. 108 SIP Trunking Mobility Mobil