The IRS has five offices within a 50 mile radius of your Zip code. You an see them if you go to:
All of them are walk-in centers, which means that you can just go there without an appointment and talk to someone. They all do have a phone number as well. Personally, I would probably call the office I wanted to go to, and explain the situation to the person who answers the phone. They will transfer you to the proper revenue agent in the collections area, and you can make an appointment. I feel this is better because even though it may be more inconvenient (you have to call ahead, you have to work around their schedules) it will save you time. I have heard of people waiting hours to see the right IRS person at a walk in office.
As far as the interest, again, that is a computer adding it to your account. If you talk to a revenue officer, they should be able to get those removed, as you do have an installment agreement.
On another issue, have you considered calling a tax attorney? Not one of those that advertise on late night TV or radio promising to settle for $0.10 on the dollar, but a real, reputable one? They do work! I had one client owe the IRS almost $400k, and they were able to reduce it significantly! To under 20% of what he owed! Now, he had a unique situation, but it was still done!
I hope this helps! And I wish you luck! If you have any more questions, please let me know.