Skype is a free Voip (voice over internet protocol) solution, use requires the ability to record and playback audio.  The program and more information can be had at http://www.skype.com/. 

Add ons such as the ability to call out and in to POTS (standard phone lines) and soft answering machines (SAM is an freeware example) make this a versatile method of synchronouse and asynchronous communication

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