IPhone 8+ calling issue

Hi all.

Ovr the weekend I visited my local apple store to resolve a hardware issue(speaker not working properly). They replaced the phone and ever since we've been unable to make international calls. Now to me that's a service provider issue. They are saying it's a mobile issue. When we call the international. Number which we've been calling for ever, we have a voicemail answers with an Asian lady. We are trying to call malta. No relation at all. Just wondering if anyone knows of this happening before and if it has any thing to do with the phone. Cheers