Greetings,
-According your phone model, search for the data-cable pinout, also purchase a data-cable (easy way) or just sold cables to tx, rx and gnd phone pins building an interface to you MCU, better if it has uart.
-May be, you need to build a 3V to 5V interface by voltage dividing.
-Now, you have an interface from you phone to your MCU. Try programming on it a simple at+command, for example, send "ATD anyphonenumber;"+<13> (CR)
This calls to anyphonenumber phone; and check the interface is working....
Best regards...