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If the amended return was submitted on July 14th (2 weeks ago), there is the strong possibility that the IRS' system had not been updated at the time the letter was generated. You may want to give it another few weeks check back with them. It tends to take the IRS 2-4 weeks, sometimes longer to completely process rec'd docs. When you call back, try calling 1-800-829-1040 if this was not the number you were calling. I hope this clarifies matters for you.
You certainly should respond to the Notice that you got proposing that you owe additional taxes for 2010. No response will often result in additional action being taken to attempt to collect.
The collection Notice will not have information that you filed an amended return.
If you can not get a response at the phone number on the Notice, send a letter and include a copy of the amended return that was filed to explain and show that you do not agree with the amount proposed that you owe for 2010.
I hope this helps to stop collection until the amended return is processed.
Please ask if you need clarification.