If I want to deploy IP Telephony to my office and use SIP trunks from my 3845 router, what hardware (DSP, etc) needs to be purchased and installed? I am a bit confused on the need of a DSP module or not. My understand of it, is that it offloads the processing of codecs from the router's CPU. However, is it required to have in order to deploy call manager express and SIP trunks?
Question #1: Yes, and you don't even need DSP resources for that. During call setup, the end device negotiate the codec. You can configure the call controller (CUCM, CME, Asterix, whatever) to limit that codec to g.729 on external calls during setup, so no transcoding is required.
Question #2: It depends on who you talk to, but I think g.729 is a great codec. It does sound a little different, but still clear and understandable. The key to g.729 is re-encodings. g.729 quality goes down when it needs to be transcoded to another codec or channelized to the PSTN. The quality really goes down if it needs to be re-encoded multiple times. For PSTN calls over the WAN (via SIP) it should be serviceable. For IP-to-IP calls over the WAN, when it can stay g.729 end to end, it is ideal.