Back to your question – toll free numbers are “designed” to be dialed within the country they are registered. It is up to the TF number owner whether to allow calls from abroad or not. Many do and, as you have guessed, they are not “toll free” in this case. So go ahead and call the number – all you have to do is add your destination country’s exit code to your 1800 doctors number (011 for USA/Canada, 00 for Europe…). Here’s a complete list of country/exit codes.
If the above doesn’t work try replacing 800 with 880. See more info.