Bell-system isolation unit. Previous to the 1960’s, Bell, who was the provider to phone lines and phone sets, did not allow any customer provided equipment on their phone lines. They required this isolation unit to be placed between the Bell Phone Network and the customer provided equipment. It was sold as protection against a fault of customer phone equipment. This isolation equipment had to be purchased, leased, and installed by Bell. The carter phone legal appeal and decision negated this requirement and allowed customers to plug in their own phone to the telephone network as along as it met FCC Certification and requirments.