rsurin: DID trunks are usually inbound only. Your order as many DID trunks as you want to be able to have inbound conversations going at the same time, plus a couple extra. You are given a group of phone numbers that are 'DID'. (You can usually convert your 'existing' trunk numbers to be DID--it sounds like you've got personal numbers already assigned, so you mihgt want to do that.) If an outside caller dials a DID number, telco picks any DID trunk to the Legend, and then sends the last 3 or 4 digits of the number to the Legend, and the Legend delivers it to the right extension. It is usually easiest if your extension numbers match the did. For example, if the DID number they give you is 555-555-1205, the extension should be 1205 if they're sending you 4 digits, or 205 if they're sending you 3 digits. That way, if one call is in progress, telco can pick another trunk, send the call to 205 again, and whatever coverage you have set up in the legend will work. In this case, your coverage will be voicemail. Does that make more sense?
I agree with Ghostin's mention of going with a two-way T1. If you need that many trunks, it might be cheaper, and you can get DS1 cards on eBay or the aftermarket folks.
Does that make more sense?