US pay-as-you-go mobile - use to call toll-free numbers?

I am going to the US soon on holidays but will have to join quite a few work calls while I am there. The calls will be all to a US toll-free number used for conference calls. I won't always be near a landline to use a cheap phone card so I'm thinking of buying a cheapish pay-as-you-go mobile phone - will this be able to call a toll-free number?

If I buy the mobile in one area but then move to a different area (same state but different area code at the start of phone numbers), will the mobile still work as a local number or will it be like moving from Ireland to England and there'll be roaming charges?

Thanks.