Years ago, I found occasionaly payphones that were privately owned that wouldn't let me make 800 calls from them - the owner wouldn't have been paid. I have some hazy recollection that new laws mandate a payment to the owner of the phone, so this may no longer be a problem.
I think all payphones in the US give a dial tone when you pick up the handset - this is so you can dial 911 without having coins. You can dial 800 numbers without coins, assuming the owner of the phone allows it or the law has changed to require it; see the preceding paragraph.
From
http://www.fcc.gov/cgb/consumerfacts/PhoneWise.html:
Be Phone Wise: Know the Rules- You have the right to place calls from a public telephone through the long distance company of your choice by dialing an access code (“dialing around” the OSP). “Dialing around” may mean dialing an 800 number, a local number that begins with 950, or a seven digit access number known as a 101-XXXX or 10 10 XXX number. Federal law prohibits blocking 800, 950, or 101-XXXX/10 10 XXX access numbers to long distance companies from public telephones.
- OSPs must connect an emergency call to the appropriate emergency service immediately and at no charge.