Easiest way for calls on same Wifi network?

Let's say that I want to call (audio, possibly video) from one Android device to another, on the same Wifi network, without creating accounts, and not every device has a SIM card. Most of the devices are Samsung. Oh, there is also an iPhone, but we can make do without that one, so an Android-only solution is enough. Are there any tools/apps for calling from one Android device to another, on the same Wifi network, without creating accounts & co.? We've been using WhatsApp but it's annoying to use, due to the impossibility to use it on multiple devices (the web version is limited in many respects) and also other arbitrary limitations. Is there an old fashion simple VoIP app that can be used for this? I don't even know what to google...