The community needs more data to help you.
Are you behind a router? Are you using an external SIP device like an ATA or an IP phone, or a softphone on your computer? Did you configure it to use STUN? etc.
Here is a guide I wrote a while back that explains the settings you could use on a Linksys Sipura device.
http://forum.voxalot.com/voxalot-gen...lkthrough.html
If all else fails, hard reset the device (if applicable) and re-enter the settings.