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Have you seen those PC telephones, they're great, the only problem with them is that your PC must be turned on for you to answer calls.
To solve this problem.
Why not create a telephone hub, a small box that sits on your desk that is just a PC telephone, it stays on all the time instead of your computer and when theirs a call it rings to let you know.
This is only really feasible with Cable, DSL etc permanent connections.
Mind you PC phone calls cost $0, you can speak internatioanlly for years and it won't cost you a thing.