[HELP] can't make my other handytone register with my * serv

hello all! i was just wondering why my other grandstream handytone 486 can’t reg with my * box but my other one registers. i have a 1u server which i’m using for testing and i’ve used centos 4.2 and asterisk 1.2.5. i have also 2 ATA’s and also using an xlite softphone for testing all are under my linksys router which i configured as a dhcp server. my problem is one of my ATA’s don’t reg, i turned on sip debug on the console and it says 401 Unauthorized, same with my xlite softphone. but my other ATA registers and can make/take calls. anyone who can help me with this? is this a NAT issue can anyone direct me to something i can read about troubleshooting problems like this? thanks in advance and more power! :smile: