Rewrite part of the example code from Adafruit FONA
I had to enter the numbers as a string. Also I found out that I had set the variables in a wrong way.

The library for the FONA Shield needs the number to call as a string. They are creating it by assigning the input from serial to a char. 
The problem was that I tried to input numbers. Also, I found out that when I tried to work with variables instead of hard coded numbers I messed around in data types.

Final conclusion: 
When calling the fona.callPhone function, you have to put in a string containing the numbers instead of actual numbers.