Upvote:2
You could have a dual-sim phone.
One sim serves the european countries, since with new roaming regulamentations you can use the mobile services as you are in your home country. If you are Danish, the only issue is that dutch customers will pay an extra to call and text you.
The other sim could be an Australian Sim card. Your clients will not pay any extra because they will call a local number. You will pay extra money when you are outside Australia to call them. But to be honest I don't think there would be a service which allows you to not pay an extra when you want to call another continent.
If you want to have a single number, it will not be free of charge for everybody.
Upvote:2
There are multiple companies that offer world wide coverage with unlimited (or nearly unlimited) data, text, and calling, without extra cost, for example Google Fi, or T-Mobile.
Those offers depend on the country you are buying them in, so you should search around for each of the countries where you go to find the best offer.
If your concern is mainly incoming calls/texts, you could also setup a Google voice account or similar from other providers, and forward it to you.