cheating again!?

Ha
My boyfriend in the past had very inappropriate pictures of his ex, now I see a number on his phone that he has called 3 times and they have called once. The number is from a place where his ex lives.. should I call the number and see if it's her or should I just have faith in our relAtionship and trust him? 
Yes I do trust him but this is really odd.
UPDATE: 
I'm getting my best friend to call because I'm pretty sure that the ex has my number.