Get a google voice number; that can forward to multiple phones. And use that number for people to call you. You can set up filters to only ring certain phones and at certain times of the day.
Consider unlimited VOIP calling plan through local Internet Service Provider.
Vonage is another possibility.
Skype is most cost efficient. Get an online number where people can call you ($18/3 months). Then have your Google Voice number ring your Skype number.
A USB headset/microphone is very important. Much better than using a cellphone with handsfree. Test the call quality with one of the numbers below:
Obtain a monitor that is large enough. 24" suggested.
Use both Windows and Linux is appropriate.
Use a 2-port KVM USB/DVI device to facilitate switching back and forth between screens. IOGEAR GCS932UB is a possibility.
Obtain appropriate VPN connections.