Mobile phones and SIP
As you already may know, newer phones support SIP (Session Initiation Protocol). SIP is a VoIP protocol supported by most programs/devices (except Skype), which means that you can use your phone to make free calls to other PCs and very cheap calls to landlines from anywhere, depending on your data plan. My data plan is not unlimited but my phone supports WiFi, so I get free calls at home/work.
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