Adam Kos Nowadays, there are quite a lot of ways to connect with the other party, and you definitely don't have to use the Phone app or even the mobile operator's network for that. Of course, we mean chat apps. Even without them, however, your iPhone offers possible ways to communicate with others if you are in an area without cellular signal coverage.
Wi-Fi calls
But of course you have to be connected to something. If we talk about Wi-Fi calls, it goes without saying that it is a Wi-Fi network. You can use Wi-Fi calls in places with weak or no mobile signal, as long as you are connected to Wi-Fi. Even iPhones since the iPhone 5c model support this function.
To do this, just go to Settings -> Phone -> Wi-Fi calls, where you can turn them on by selecting the option at the top. When you then have Wi-Fi calls available, you will be informed about it on the top line of menus, right next to the name of the operator. This is a clear indication that the subsequent call will be handled over the Wi-Fi network. You can also turn on the option Add Wi-Fi calls for other devices here, so you can then call from your iPad or your Mac.
HD Voice/HD calls
It may be obvious from the name itself that this designation has to do with the quality of the transmission, not so much with its technology. HD calls are supported by virtually all modern phones, and this function is designed to remove noise from the transmission itself. It is therefore closely tied to support from the operator, but in our case all three provide it. The problem here is in the codec used, where compared to the previous one labeled AMR-NB, AMR-WB with a significantly wider frequency band (50 to 7 Hz) is now used.
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Pavel Jelic Volta
This is essentially a similar matter as in the case of Wi-Fi calls, but here the call takes place over the data network, usually even in places where the signal is poor. The service stands out for its fast connection, which takes place within two seconds. Here too, support from the operator is necessary, with us it is again provided by all three. But even if you call via the data network, you pay for the call in the same way as if you were calling in the classic way. VoLTE typically does not work in roaming and you can surf the Internet while making a call.
VoIP
Voice Over Protocol is actually an internet call instead of a typical landline call. With the help of special protocols, it digitizes your voice and finds its use in practically any data connection, be it mobile data, Wi-Fi network or where it is most used, namely in the company intranet.
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Vratislav Holub 