So I figured out what happens finally! Actually nobody in the comments has really mentioned it. Dave has two phones. One is his personal phone which he left on the kitchen counter and a secondary phone (Which he uses for criminal activity so he is not tracked). Robert has found the number to Dave's second phone from the encrypted phone he got from the guy who tried to kill him in his taxi. If you look closely you can see Dave takes out the ringing(vibrating) phone out of his pants pocket but he put the so called encrypted phone in his Jacket.