iPhone users can use "FaceTime Audio", a free voice call service operated by Apple. The amount of data is small because it is only voice, and it is overwhelmingly cheaper than the voice call of the communication carrier. Communication costs will be incurred if you use a mobile line, but even with a Wi-Fi connection, you will not incur even that slight communication cost.
Similar services include FaceTime, but FaceTime audio is voice only. The camera is not used, so there is no need to dress yourself up before calling. Any iPhone that runs iOS 7 or later can be used, so as of 2020, most iPhone users should think that FaceTime audio can be used for calls.
When calling with FaceTime audio, enter the other party's mobile phone number or email address in the FaceTime app. In this case, the other party is surely using iPhone, and you must specify the mobile phone number/email address registered in the other party's "Contact information for incoming FaceTime".
However, it's a good idea to ask the person you're about to talk to if you're using an iPhone and which email address you have for FaceTime calls. Open the screen in the order of "Settings" → "FaceTime", check "Contact information for incoming FaceTime", and it seems to be troublesome.
That's where "contacts" are used. When the page of the person who registered the name, telephone number, and email address is displayed, an inquiry is made to Apple's server, and if that telephone number / email address is registered for FaceTime incoming, the line "FaceTime" appears. I will. If it is a phone number/email address that is not registered, it will not appear, so it will be a factor to judge whether FaceTime (audio) can be used.
After that, if you tap the handset button displayed on the FaceTime line, the call will start and the information of the other party will be displayed on the screen of the phone application. You can also register in "Frequently Used Items", so why not try using it?