They're are a lot of settings to make SIP work on the IPECS, so it's hard to say what may be going wrong. Most SIP carriers have specific instructions for each type of hardware, do you have any information available from your carrier for the IPECS?

How is your "SIP User ID Attributes" setup? I'm assuming you're just using attribute 1 for all of your extensions? The first 3 settings are generally something like:

Registration User ID: username@ipaddressofSIPserver
Authentication User ID: username
Authentication Password: password

A big one that is often missed is under "Station Attributes". Each extension needs to have the "SIP User Table Index" set to 1, or whichever SIP User Table you're using. If it is set to "0" (default), you won't be able to dial out.