Cisco Ata sip based

Hi Group.
Recently i bought two cisco ata for trial purpose but now i am not able to configure it i have got some question.

1. Do i need a Gateway .
2. if not then how to configure the phone numbers and call because both ata will be in the same lan.

3. Do i need to use SIP Based Proxy server . for that purposes.
if yes which ones , i need free sip based proxy server .

Please do help me out is urgent