On August 10, 1945 the Japanese notified the Swiss government that they wished to surrender and requested that they act as intermediary with the United States.

After much internal struggle and argument, the Japanese Government instructed its Minister to Switzerland to advise the United States through the Swiss Government that the terms of the Potsdam ultimatum would be accepted if Japan's national polity could be preserved.

This would not have been a telephone call. It was most certainly communicated via telegram.

Source: http://www.history.army.mil/books/wwii/MacArthur%20Reports/MacArthur%20V1/ch14.htm

The United States acknowledged this on August 11th and suspended most military operations on the 12th. There was a bit of confusion and the Japanese officially surrendered on the 15th.