iam runing SER

and my client is registering with Asterisk, running call center

he keep getting the following error

chan_sip.c: Forbidden - wrong password on authentication for INVITE to '"M0601001458000013456" <sip:user@domain.com>;tag=as5bbc2eec'

and voice going choppy, when he call from single user the voice go clear

when he intiates the calls from number of people voice is very choppy

and some time not able to hear

all are public IP, not NAT here

any suggestions

ram