The Internal Revenue Service has offered answers to the four most frequently asked questions about Estimated Tax, they are:

1. Is an S-Corporation required to pay quarterly estimated tax?

Rarely does an S corporation make estimated tax payments.

An S Corporation must make installment payments of estimated tax if the total of these taxes is $500 or more:

The tax on certain capital gains,
The tax on built-in gains,
The excess net passive income tax, and
The investment recapture tax.

2. How do I know if I have to file quarterly individual estimated tax payments?

If you owed additional tax for the prior tax year, you may have to make estimated tax payments for the current tax year.

You must make estimated tax payments for the current tax year if both of the following apply:

You expect to owe at least $1,000 in tax for the current tax year, after subtracting your withholding and credits.
You expect your withholding and credits to be less than the smaller of:
90% of the tax to be shown on your current year’s tax return, or
100% of the tax shown on your prior year’s tax return. (Your prior year tax return must cover all 12 months.)
There are special rules for:

Certain taxpayers with higher adjusted gross income
Farmers and commercial fishermen
Aliens
Estates and Trusts

3. Do self-employment taxes need to be paid quarterly or yearly?

If you are required to make estimated tax payments, self-employment tax is paid by making quarterly estimated tax payments which include both income tax and social security tax.

4. When are the quarterly estimated tax returns due?

You only make estimated tax payments using payment vouchers. There is not an estimated tax return.
Your first estimated tax payment is usually due the 15th of April.
You may pay the entire year’s estimated tax at that time, or
You may pay your estimated tax in four payments that are due April 15th, June 15th, September 15th, and January 15th of the following year.
If the due date for making an estimated tax payment falls on a Saturday, Sunday, or legal holiday, the payment will be on time if you make it on the next day that is not a Saturday, Sunday, or legal holiday.

If you have a specific question about your Federal Tax return you can call the IRS and receive Live Telephone Assistance. Here are the particulars:

Telephone Assistance for Individuals:
Toll-Free, 1-800-829-1040
Hours of Operation: Monday – Friday, 7:00 a.m. – 10:00 p.m. your local time (Alaska & Hawaii follow Pacific Time).

Telephone Assistance for Businesses:
Toll-Free, 1-800-829-4933
Hours of Operation: Monday – Friday, 7:00 a.m. – 10:00 p.m. your local time (Alaska & Hawaii follow Pacific Time).

source: irs.gov