This "free" phone service is available to anybody with an internet connection, using modern digital audio technology that streams your voice from your computer or cellular phone to another user.

    And there are several ways this can be implemented by any person, or persons -- using readily available technologies ranging from very simple (e.g. using your PC with free downloadable software and an inexpensive mic/headset) to more elaborate methods that include a digital telephone set (which may be bought used in working order from commercial surplus outlets starting at around $25 each) that simply plugs-into your internet connection (though you'll have to configure it yourself with a little help from a tech-blog here and there), and better still -- installing Asterisk on your computer.

    Asterisk is an open-source (free) PBX system just like those in various businesses that answer your incoming call automatically, and a voice tells you "if you know your party's extension you may dial it at any time..." (and yes, it's free but you will have to configure it yourself; you probably can't afford what it costs to pay somebody else to do it). One might find it overkill to have such a system running at home -- the best reason to have such a system however is that it screens your incoming calls better than any answering-machine/voicemail setup, and I've found it to be the ultimate in dealing with telemarketers.

    Without any further digression, here is the main "catch" or pitfall: if you want to dial an actual telephone number, your free SIP provider can do this for you -- if you pay for the service. And this isn't really a catch; it just isn't free. (It's also a lot cheaper than what one pays their local telephone company. Customer support also isn't as diligent; a good example of this would be Vonage, and MagicJack.) If you can get your friend in Canada, the Philippines, el al to buy an inexpensive headset they can plug-into their PC, and install a software app for SIP communications, it shouldn't cost anything more than your existing internet service and the price of a mic/headset.

    There is yet another free option that utilizes your cellular phone. (I must also mention that it needs to be a smartphone of some sort -- either an Android, or iPhone, because it requires an app specifically designed for SIP communications.) If you use this method of SIP communications, be sure to use it only while connected to a WiFi network, else you'll burn-up your monthly data allocations rather quickly (unless of course, you have an unlimited data).

    So, what exactly is 'SIP?' It stands for 'session initiation protocol' -- and unless you want to know how a watch is made, just stick with what time it is, and move on (i.e. it's probably more than what you need or want to know, other than how to call the other side of this planet without paying for it). There are SIP providers everywhere, some of which are primarily focused on providing telephone service for businesses, while others are mostly not for profit organizations (though the latter will still charge a subscriber for connecting their outbound calls to the public telephone networks by which one would dial an actual phone number, because costs are incurred from such connections and somebody has to pay for it).

    And while commercial SIP providers are more interested in getting you to pay them for their services, there are many who will still provide you with a free account to do all your internet calling -- along with letting you make calls directly from your browser. (If you have a laptop, or your desktop PC has a built-in microphone, you don't have to buy anything. The mic/headset is still recommended though, just for consistently clear and readable audio.) Be aware that there are some SIP providers who will only give you a few minutes of talk time on a "trial" basis, and will soon expect you to pay for monthly services rendered, else they will disable your "free" account. (One such provider is 'SIP.US' -- they are not apt to find popularity among the casual SIP user.

    Free SIP Services, and SIP Resources include Linphone (so named after the Linux operating system that was a prominent component of the development of 'voice-over-IP' technologies), AntiSIP, and SIP2SIP just to name a few.

    A commercial SIP provider here and there will accommodate the home user too, only charging for their service if one decides to subscribe to their PSTN (Public-Switched Telephone Networks, such as ATT, Verizon, et al) connection services, and it isn't uncommon to find a provider here and there that will provide a number of services free of charge.

    One such company I have encountered is OnSIP. Sign-up or their free SIP service and among their free services are multiple extensions for friends/family, so one person with one account can connect with several others using one account. (The others will still have to create their own passwords, and login from their own computers/phones; though using IP phones, such as a Polycom IP 450 makes SIP-calling as easy as using the phone on a land-line.)

    I should also mention Google Voice (even though they're Google... ), they provide a free phone number that connects to your mobile phone or land-line -- if you want to use that phone number without linking it to your phone(s) they will provide you with an actual working phone trunk if you pay to subscribe to it. And it is also likely less expensive than ATT, and the rest. That one is up to you, and what you're looking for when it comes to a phone. It's your call.