Have you called your phone company to see how much it is to add international service? It might be very low. You just have to make sure it is not a monthly fee PLUS minutes. I know you said you want it to be free, but if it is just $5 a month extra and you can cancel any time you might consider it.
I’ve heard of google voice, which I think the other person does not need to have it, but there is a $10 fee I think. I would have to research it more for you if you are willing to pay. Hopefully, someone else has a free suggestion.
I use Whatsapp, but you are right that for business it isn’t going to work well unless the other businesses all use cell phones. I use Whatsapp for business. All of my reporters and contacts for work use whatsapp in the UK, Italy, Kenya, Germany, it is very widely used in the UK. I think my boss uses an additional service for international calls, I can ask her tomorrow, too late now, she won’t be awake.
If the business has an email you can get in touch that way and then ask if you can WhatsApp, skype, or zoom with them.
I probably wasn’t much help.