First, I did call him back, but apparently he is also pretty busy and couldn't take my call right away. For whatever reason, my phone did not ring when he called back a few minutes later despite that I was sitting by my phone with the ringer on maximum. I have no missed calls on my phone, but somehow there is another voicemail from him from after I returned his original call. I think this is possibly because I screened the first call due to it being from a restricted number, so my phone took the liberty of deciding the second call was not important. I definitely had adequate reception and was literally sitting at my phone waiting for it to ring when I missed the second call.
Cell phones are awful at being phones. To make matters worse I live in a dead zone and can only receive phone calls on that number when I'm not at home.
I work at the medical school 4 days a week.. worst case scenario I will just go to his office.