Media for Phone to VOIP

Phone to VOIP

Hi all

I am an awful story teller so I'll get right to the point. If someone calls on a specific number, and is for a specific problem, the Secretary, after answering can divert the call to someone else, but the tricky part is to use a "Voice over IP" for inside company, but Incoming call will be from a regular Phone. I was thinking on using Skype for inside (with USB Phones), combined maybe with some kind of "Phone Answering Software", but don't now how that can be connected to something like a Skype, or even if this is possible. What can I use ? Besides "SkaypeIn" if possible.

To make it shorter, how can a VOIP application can be "combined" with regular phone line.

Thanks