The BT-327 is used when you have a telephone using two prong handset or headset jacks. Normally older phones or switchboards using this type of jack have two sets of the two prong jacks, one on the left and one on the right. Since only one jack is used for the operator, this adapter allows you to convert the unused jack to a standard modular handset/headset jack allowing devices such as Omnicron "TSA" series adapters to be used with your phone. $20.00 each.