AT&T has at least one MPEG-4 patent in MPEG LA (Score 1)

The MPEG LA January 1, 2006 MPEG-4 Visual Patent List lists patent
6,134,269: Fixed or adaptive deinterleaved transform coding for image coding and intra coding of video.

The inventors are from AT&T and Lehigh University (Competitive Technologies acted as Lehigh's agent and submitted the patent to the pool). So it looks like AT&T might have at least one patent in the MPEG LA pool (just through a related entity).

some other potential AT&T patents:

4,999,705 Three dimensional motion compensated video coding
5,227,878 Adaptive coding and decoding of frames and fields of video
5,253,056 Spatial/frequency hybrid video coding facilitating the derivatives of variable-resolution images
5,270,813 Spatially scalable video coding facilitating the derivation of variable-resolution images
5,500,678 Optimized scanning of transform coefficients in video coding

Some of these are prior to MPEG-4, but you can follow the "Referenced By" link on the individual uspto patent pages to see other later patents.

I have seen AT&T patents assigned to:

AT&T Corp
AT & T Corp
AT&T Bell Laboratories (old, but still valid patents)
Bell Telephone Laboratories (patents mostly? expired)