Umm probably because they don't answer the phones then and are not seeing patients till 8:30 or 8:40. That way they have time to be ready in the morning and time to finish up loose ends at the end of the day without being interrupted.
If the hours are 8-5 then the phones should be answered during those times; however, like most general company's medical offices are also short handed. They generally arrive PRIOR to the posted hours to get the day ready and generally do not walk out the door at exactly closing time. My Dr's office generally calls me back within an hour if I don't call right at closing time, then I get a call the next morning.
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