I have Comcast at my home and I can assure you that Comcast does not block SIP ports. But, the router at your office may be the culprit. Our office router once allowed me to use my SIP softphone to connect to my * server at my house. But then when the IT guys installed a new PIX system, it blocks everything unless they specifically allow it.
What I have found over the year or so I’ve been using * is that some hotel and airport wireless setups block SIP. Again, they are probably not blocking SIP ports individually, they just allow only certain ports through and anything else is dropped. The hotels have an incentive to block SIP traffic because it forces you to use their long distance service. Hotel and Airports probably block it to reduce traffic.
In any case, I think you will find that Comcast is not the blocker when it comes to SIP.