Phone calls from my modem

[Len]

I've asked this question before but have lost the answer.
On my last phone bill, i was charged for two calls to
Chile which I queried with BT. They tell me that the
number is a data line which has been accessed from my
modem using international dialling which terminates the
call to my ISP and connects via the international number.
How do i protect against this happening again?

Many thanks