They are actually charging you for forwarding a call. When you don't answer, the incoming call will be forwarded to your voicemail, which is a US number. So it would be the same as if you were to call your voicemail with your own cell phone.
I found this out when I was working in Canada a while back. I told my girlfriend to call me on my cell and I would call her back with a phone card. They charged me for every call that I didn't answer.
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