You should look into the various SIP compatible VoIP providers that are out there. Some providers will offer 2 concurrent calls (meaning a line can accept two incoming calls at a time), other providers offer more or less concurrent calls (sometimes called "Channels"), and some charge a fee for extra. Essentially it is entirely up to the provider and the phone plans they have available for you to choose from. I for example have a phone number that can accept up to 10 incoming calls on it at a time, and I only pay $1/month for the privellege (this offer is no longer available from the service).
Once you have the VoIP account, you should download and install NCH's Axon PBX software. It will manage the incoming calls for you and ring the appropriate phones/extensions and transfer the call to voice mail when necessary, etc.
You will need either VoIP softphones that run on your computer (Express Talk), SIP phones that run stand-a-lone (connect to your network directly via Ethernet), or ATA adapters (to connect regular analog phones) to interface them with Axon and receive/making calls through your VoIP service.