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Can you buy a landline phone that will allow apps? I use a phone card app to call abroad quite regularly but calling from home with a mobile never has the best reception,
Get an account with someone like sipgate.
You can then call from your mobile via wifi, your PC or buy a voip phone
Or call via WhatsApp over WiFi for free.
There have been a few DECT phones that run Android. Though most seem to get crap reviews.
eg Gigaset. https://www.amazon.co.uk/Gigaset-SL930A-Premium-Phone-Android/dp/B00GV9R26E/
We have call over wifi on most of our mobiles, so as long as you have a decent wifi router (and network speed) mobile works well.
How about an old mobile with no SIM on WiFi.
If your plan is to use your existing phone card app to make calls over the landline, I don't expect that would work - the phone system is different. But if you just want to make calls to foreign places using your landline, the suggestions above look good.