In almost all of the Whitlock docs there is a parts list and simple schematic to make the troubleshooting test leads. For RCA connectors they are model TA-R1 and also available from Jensen Transformers for about $10 for a pair and a troubleshooting manual.
Starting on p13 of the
Understanding Finding Eliminating Ground Loops in Audio & Video Systems and on p.5 of the
Troubleshooting Guide there is a parts list and schematic.
Jensen Transformers, Inc.,
9304 Deering Ave., Chatsworth, CA 91311
Toll Free Order Line: 866.476.6291
end_of_the_skype_highlighting Voice:818.476.6291 WELCOME TO JENSEN TRANSFORMERS, INC.
As far as AT&T, simply call and tell them you need someone to check the ground. You don't have to make up a story! If they use a rod, insist it be common bonded. Its incombent upon them to verify the configuration.
As far as the procedure, follow the 10 page
Troubleshooting Guide 'to the letter'! Its not difficult!