Comcast Corp. (Nasdaq: CMCSA, CMCSK) is still trying to trace the root cause of a technical issue that left some of the MSO's residential and business phone customers without phone service for a 90-minute period on Tuesday.

The MSO confirmed that a number of customers were unable to place phone calls yesterday from 11:00 a.m. to 12:30 p.m. Eastern Time. During that span in Denver, some customers could get a dial tone, but were met with a message that "all circuits are busy" when trying to place outbound calls.

"Services were quickly restored and we are investigating the cause so we can take steps to ensure this doesn't happen again in the future," MSO spokesman Charlie Douglas said in an emailed statement.

Comcast did not say how many customers were affected or which regions of its networks were down temporarily, but it was short of an across-the-board outage. Comcast ended the first quarter of 2009 with 6.76 million digital voice customers.

The MSO has yet to pinpoint any specific equipment or software issues, but, depending on the service region, the company uses softswitch gear from two key suppliers: Cedar Point Communications Inc. and Cisco Systems Inc. (Nasdaq: CSCO).

— Jeff Baumgartner, Site Editor, Cable Digital News