Does Android not have the equivalent to FaceTime audio? I get that for x-platform you have to use one of the apps being discussed. I use FT Audio with my sister, who's in UK, all the time (I'm in Chicago). Completely free and excellent sound quality.
Google surprisingly has a raft of telephony options.
You can use Google Duo to make voice or video calls for (other than data costs) free, Google hangouts also has voice-only plus video options and of course Google voice integrates with the classic telephone network and has cheap international rates.
Google Fi has free calling from the US to over 50 countries and otherwise their plans start at one cents a minute depending on destination. https://fi.google.com/about/unlimited-calling/
Most of my friends from Asia tell me WhatsApp was and is popular because it carried voice over data, bypassing the PSTN which apparently has very high per-minute rates.
If you want to go slightly higher tech there are telepresence appliances like 8x8, Amazon or Google IOT devices or you can just use sip phones and call between the devices free of charge using your own pbx software or a free service like Callcentric's IP Freedom plan.
There a million options that either let you opt out of Facebook's data collection and trade it for Google's, or just opt out entirely.
Google was pushing Hangouts heavily for a while, and I think that's still bundled with Android but is now on the way out. It did the job last I checked.
Android (at least used to) has native support for SIP through their phone application. I used it quite a bit 5 years ago or so, but moved over to...well, I can't remember. A 3rd party app that gave better visibility over what was happening with the service. I don't use VOIP too much any more, Signal is fine.