Asterisk-1.4.15 issue - busy signal only

Need help!!!

I installed asterisk-1.4.15 on Fedora core 6 and all my BT102 SIP phones were registered to asterisk, but when I called one extension, I got only a busy signal.
On the other end, I can call the extension for the voicemail, for example :

exten => 250,1,VoiceMailMain() , and hear the pre-recording voice message

Please help me

Thanks in advance