Strange Number...

So me and my boyfriend were watching netflix the other day and his phone rings and its a message from a number not saved... He didnt answer it or ask who it was. He wouldnt let me see his phone (but usually he will let me see it, I know his password and he knows mine). And then about 30 mins later same number texted him again saying "do you want to see me today." I got pissed. He said he has no clue who the number was, and then he called the number and on the voicemail it was a girls voice. She called back and when I answered she hung up the phone so quick. This has happened in the past with him and he was cheating on me... But this time he's saying he didnt do anything.. Do I believe him and just drop it? Help please!! 😓😓😓