In reply to Hillseeker:
The open standard is SIP. Sip calls can use a software phone or a hardware sip phone (available on ebay etc) that looks like an ordinary phone.
You'll need a SIP provider, and you can get free incoming SIP with a UK landline number from sipgate.co.uk which includes voice messaging sent via email when you receive a message.
On Android I use Sipnetic as a software phone (search google play). Android has stopped inbuilt SIP support sadly, though if you have an older phone you may still have it. I can't comment for iOS.
https://linphone.org/ is a free open source softphone that works with sipgate.co.uk and is windows, iOS, linux etc etc compatible. You'll need a mic and speaker, though a headset with mic will do. You can also buy USB handsets (I used one for years)
Most chat apps (e.g. Telegram) now have inbuilt phone/video phone too which is good for friends and family who are also on the chat app. That's how we communicate with our kids - works anywhere in a world with WiFi (except maybe China!) They only call "old school" if we're in a non data area (i.e. Scotland!)
Having Sipnetic on my mobile is the most convenient use of SIP as it rings and is used just like a normal phone call. Add the chat app calling facilities and we no longer have a land line number at the house (The SIPgate landline number is used for business.)
I hope that helps,
Steve