Asterisk 1.4.1 sip problems

I don’t know why the new asterisk make problem with the sip phones, when I was using the billing system from the sip phone asterisk wasn’t able to send back the voice to sip phone, on the asterisk consol you can see that the sound back was failed, couldn’t make frame or some thing like that, the billilng working fine, but you can’t here any thing, asterisk block all sounds