First of all, consider that a typical smartphone makes three types of external connections:
1) Voice phone call, using carrier cell phone network, using GSM (typically)
2) Data connection, using carrier data network
3) Data connection using WiFi (private short-range 802.11 local network)
On any smartphone, connection #1 is only made one way…using the native built-in phone capability of the device, typically using a SIM card to identify who the user is.
Now here is where it gets blurry:
On an iPhone, for example, you can load the Skpe App and make calls. However, it’s just an app….so it is only using connections #2 or #3. Since, typically, data traffic gets less priority on the carrier’s network, the quality/reliability will not be as good. Plus the app is typically not 100% integrated into the phone, so things like dialing, ringing, and so forth can work in non-standard ways.
The curious app on the iPhone is Google Voice. Google voice actually uses connection #1, however the way it works is that the app reaches out and dials into a Google voice server in your area. However the text and voicemail services depend totally on connection types #2 and #3…they are just data.
Now to confuse things a bit are things like Cisco ‘hybrid’ voice-over-wifi solutions.
Once a company has a wireless LAN in place, it’s logical to want to have wireless handsets that can use the same network for voice.
So instead of giving every employee a $400 Cisco WiFi phone, they just load an app so the user’s smartphone can work the same way.
Your cell phone becomes a WiFi phone extension for the company by running this app. But it’s still a cell phone, and the cisco wifi stuff is separate and distinct from the GSM phone company connection.
To further confuse things, there are devices known as GSM gateways used to save cost by forwarding landline calls to a GSM attached gateway….it’s much cheaper to call from GSM to GSM versus fixed line to GSM….but I digress.
Years ago, a company called SpectraLink introduced the first WiFi phones. They made (and still make) WiFi phones designed to work with whatever business phone system you have.
The tricky part to all this is getting the required PBX features to work on a wireless phone connected to a company wireless LAN, and of course having a wireless LAN that has the speed and coverage to make is suitable for voice communication.
If you have a Cisco VOIP system, then you would most likely use Cisco wifi phones (or apps), while you would need a different solution (like SpectraLink) if you have an Avaya or Nortel system.
By the way, these work on old-fashioned PBX phone systems too, it does not have to be VOIP.