Conspiracy requires an agreement between two or more parties for an unlawful purpose.  An overt act in furtherance of the conspiracy establishes evidence of the conspiracy even if the overt act itself is lawful.In other words, evidence of the conspiracy to buy drugs in this particular case could be established by one party crossing the border with illegal drugs and the telephone call made to the other party would constitute an overt act in furtherance of the conspiracy.  A telephone call itself is a lawful act, but in this case if it is made for the purpose of buying drugs, then the telephone call is an overt act in furtherance of the conspiracy.