Peer to peer calling


I have been using Asterisk within our company for the past couple of years and has been working out well. Internally, we use SIP-based CISCO phones. Externally, we have PSTN connectivity.

I have also tested that SIP software clients such as Windows Messenger can be used to reach our Asterisk PBX and make outgoing calls. Obviously, this requires username and password.

What I am wondering if there is any software application that an external person can use to reach our PBX over the Internet (without the need for PSTN connectivity). The caller then gets to hear the standard menu options.

This may be a simple question for you. There is so much information about Asterisk that I cannot find answer to this simple question.

Thank you in advance for your help.