When should I contact again after an internship interview?

About a week and a half ago I had a phone interview for an internship and thought it went really well. The interviewer told me that I would hear back from him within 2-3 days. On the 2nd day he emailed me and said that I would hear from him within the beginning of the following week. In the middle of the week I emailed him and asked if I would still be hearing from him. I received a phone call from him a day later but was not able to return it until late at night, so I decided NOT to return the call and instead called the next day and found out the interviewer was off until Today. Left a message and was told by the person on the phone she would let him know I returned the call. I'm really, really interested in this internship and thought I at least had a chance for at least an in-person interview. It's fairly early in the day and who knows when he came into work today, so when should I try to contact him again? I'm uming he isn't going to return my call first thing in the morning or even during most of the day and probably leave it til towards the end of his shift... so should I just wait and hope he calls back by the end of the day? Or call around 4-5 and call him then? I don't want to pester. I'm leaning towards just waiting and not calling again.. I think he may just be busy or something.