AT&T 2Wire Routers
AT&T Supplies it's U-verse customers with their own 2Wire modems. For the most part those modems work well with OnSIP>
A new version of firmware is being pushed to ATT U-verse 2Wire modems. The new firmware turns on the SIP ALG and prevents users from accessing the advanced tab to turn it off.
The only remedy is to have the modem placed in bridge mode and add a recommended router which does not have a SIP ALG.
External links to turn the 2Wire router into bridge mode and how to setup a Linksys router to work with a 2Wire.