Thanks.
I just registered for a NetTalk mobile account. I believe the SIP credentials (username/password) are the same ones input during the registration process. That said, what is left is the SIP server for NetTalk mobile. If anyone out here knows the SIP server for mobile NetTalk and don't mind to post it here, I can give it a try with an Asterisk and/or FreeSWITCH PBX System to register my newly NetTalk mobile account.