Yeah - as the other guy said. Look into Google Voice. I use it (not internationally though). Wifi calling yes - but you can also add phone numbers for it to call - and maybe it can (for a smaller fee) redirect calls to your international SIM phone#.
Somebody used to build a phone that had two SIMs in it. It could operate on a dual phone#. It was long ago so I can't remember who - but you had multiple phone# and could switch between them.
Google Voice operates like a PBX - people call the virtual number and it redirects the call to a list of real phone#. At the very least get your vm in your inbox - transcribed as text too!
I use the free edition here in the USA to simply redirect calls to my landline & cell# (so mom only needs to call one number). And it has terrific spam call filtering. It also lets you spoof your caller-id when making outbound calls so that the virtual# appears (great for calling the bank). I'm an iPhone person - but I hear it has better integration on Android.
Although - the SMS UI sucks. And not all services can SMS to it. As a "chat" app it stinks. But it is one platform for all messaging & calls. Cell phone, web browser, landline. I believe there are business plans - and for making direct calls as Voip you can pay per-minute fees.
Check it out - it might work for you.