Well, I guess my feelings would depend on several things. Is this the first time your T has never called you back? Voicemail isn't a 100% guarantee. I know that I have received voicemails at work that cut off, and I even accidentally erased one before. For me, my T has always called back within several hours. I rarely call her. I've probably left her 3 voicemail messages in a year. She told me when I started seeing her that she checks her voicemail several times per day and calls back within several hours usually (unless it is late at night). So, if this happened to me, I probably would have left a second message two days after the first message and would have assumed that she didn't receive it. I am not sure of your situation or T relationship, but you might want to consider calling again and bringing this up with her the next time that you see her. Unless she did not receive the message or had stated she was going on vacation and given you the name of another T to contact in the meantime, then i would find this behavior unprofessional and unacceptable. I hope that you get things sorted out with your T.