It's probably CenturyLink, which is the 3rd largest LEC behind AT&T and Verizon. (Comcast isn't classified as a Local Exchange Carrier, it a cable operator.)
It definitely is CenturyLink. Their Level 3 interconnects have been saturated for awhile, longer than March.
AT&T and Netflix signed an interconnection agreement, so LEC three must be CenturyLink (who also announced a 1Gb broadband trial in Portland)