Yes the 500 can do it. You dont need the voicemail to make this happen. If you are using a PRI (DID) the you simply route the calls based on CLI or DID to the fax server. If you need the number dtmf'd to the fax server just have it repeat CLI or DID and it will send the digits received to the fax server.

Yes you need to have a modem and fax server attached to the sli group or station. Unless you have a vpmf module in the voicemail.