Voice over IP - performance

Does anyone have any real-world figures for VoIP performance on various platforms? In other words, how many calls can an otherwise unused e.g., Cisco 2600 be expected to handle if it's the conversion point from trunked voice calls to IP. Some rough numbers for different codecs on different hardware would be very useful. Most specifically I'm interested in Cisco router platforms but other vendor stats would be appreciated as well. Off-list replies please; if anyone is interested in the results I will condense the responses and post back to the list. Thanks, C.