Blocked Phone Number iPhone

You did not block the number - there is no way to block numbers on the iPhone. The doctor has blocked his/her number from being visible (masking the caller ID information from the recipient of the call can be done by the service provider for all calls, or for a specific call by dialing *67 before making the call). Different phones display this as Unknown or Blocked.
The fact that your phone is not ringing is a separate issue. Although other phones have a feature to ignore calls from unknown/blocked numbers, the iPhone does not have that feature.
You might try resetting your iPhone. To reset the iPhone, press and hold the home and sleep/wake buttons until you see the Apple logo (ignore the slide to power off prompt).
17" CD MBP, iPhone 3G, iPodTouch Mac OS X (10.5.6)  since the e

Thank you for your timely response. There is no doubt that I did something to block this number. I remember doing it, I just do not remember where that was, only that it was a reflexive action I really didnt want to make. That being said - I understand that a formal blocking procedure is not available on the iPhone. Everyone says so. I can not find one now. However, I did something, the end result of which is that that caller is blocked. I see a blocked call message on my screen. Since this is my physicians office, I see no reason they would have blocked the call from their end. They are trying to give me important information. I have reset my phone per your suggestion and when I can get in touch with my party I will ask them to call me again to see if this has solved the issue for me. If it does, I will post it here. I should have thought of a reset myself.
Thank you again.
Alum. iMac 2.4 Mac OS X (10.5.1) 1Ghz RAM

How do you know it is your doctor?
Anyways. Just like everybody says, there is no way to Block a caller from calling you. The blocked means that there is no caller ID information associated with the number. For example my sister has put blocking on her home phone so whenever she calls me it says, Blocked. I can still answer, she can still leave messages, and so forth.
A lot of offices have multiple lines and the number you call in on will not necessarily be the line that they call out on. Perhaps to avoid people calling on lines that arent for in-coming calls the lines are blocked? In that case it could be something that was specifically set up. Or it is possible that the phone company just set up a business switchboard by accident with those settings. In any event it is not at all uncommon to have businesses show a blocked number.
iMac 2.8 Ghz, 4 GB Mac OS X (10.5.6) 2.5 iPhone (8 GB 2.2)