From FAA ORDER JO 7110.65X. This should put the matter of RT procedures to bed.

2−4−3. PILOT ACKNOWLEDGMENT/READ BACK

Ensure pilots acknowledge all Air Traffic Clearances and A TC Instructions. When a pilot reads back an Air Traffic Clearance or ATC Instruction:

a. Ensure that items read back are correct.

b. Ensure the read back of hold short instructions, whether a part of taxi instructions or a LAHSO clearance.

c. Ensure pilots use call signs and/or registration numbers in any read back acknowledging an Air Traffic Clearance or ATC Instruction.

NOTE− 1. ATC Clearance/Instruction Read Back guidance for pilots in the AIM states: a. Although pilots should read back the “numbers,” unless otherwise required by procedure or controller request,