Intranet dial-in server (NO PUBLIC LINES)

I work at a computer store as a repair technician. We recently switched over to VoIP phones leaving us with absolutely no public telephone lines. My question to all of you is… How can i set up my desk computer (I am the software manager. I get a desk HEHE!) which runs gentoo to serve as a dial-in server allowing its clients to have access to the internet along with providing a !DIAL TONE! (VERY IMPORTANT) for clients waiting to connect.

Tux will always reign supreme!!!

Depends of your VoIP provider. Some VoIP providers are giving numbers for direct incomming calls.
These phones will behave like normal PSTN phones.
If this is the case, and you wish to make additional services - then:
Install Asterisk 1.2.1 - no problems with Gentoo.
In your sip.conf you should put registrer - server, username etc - see example in sip.conf
For every registration you should create extension.
Then when somebody calls your number - adequate extension will be envoked and this way you can receive calls to Soft or hardware SIP phones.
Other solution is if you have static IP, then you could negotiate with your VoIP provider to direct all calls to your IP.
This way - if you have 5 numbers from your VoIP provider, you could accept up to five calls on same number - and Asterisk can do other part - but this is more complicated solution.