We sell Carrier Services and usually size based more on the number of calls not the number of people. 30 simultaenous calls should be easy on any good (read: true business class) Internet circuit. @scottalanmiller has the bandwidth nailed on the head above. We di try and get users to get the best Upstream bandwidth which for cable modem installs around here is very difficult (Time Warner only offers 5Meg UP even in Business Class which is pathetic).
For our needs internally we needed well over 100 simultaneous call paths. We went with a dedicated circuit to a Rochester based cough carrier cough and a backup trunk over an Internet circuit. While bandwith has never been an issue lets just say I can't wait for the contract for the cough carrier to expire so that we can port and go to the Internet for our needs and dump them!
Wow, this is kind of right along the lines of what I've been researching on remote extensions...
One of the things that we routinely do is to block all non-free calling so even if our extension, PBX or platform were to be hacked, even if they gained access to our SIP trunk, they can't rack up long distance calls or whatever.