employment verification - DirecTV

I worked for at&t for 13 years, since it was PacificBell in Los Angeles.

2 questions:

1. Both of my previous managers from both of my positions changed their contact numbers and I need to know what number can I put for potential employers to verify my employment at at&t?

2. Also, how can I contact HR to ask them what they have as my reason for leaving the company on my last position there?

PLEASE HELP because I'm having issues with employment verification from 2 potential employers with this.

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