Problem with installation OD Asterisk

Hi, I just installed Asterisk on a computer. When configuring the phones that Ip have Denwa brand these are not registered in the central. On the phone he just keeps trying to register. I have followed the help but I cannot configure the phone. Any help you can give me?

Does any other SIP phone work with the same username/password?
Do you see anything (Warnings,Errors) on Asterisk console when the phone tries to register? Set verbosity to at least 3.
BTW, What version of Asterisk are you using?

If it just keeps trying then more likely a network issue (Your phone is not able to reach Asterisk)