Telephone utilities shall furnish and maintain adequate plant, equipment, and facilities to provide satisfactory transmission of communications between customers in their service area. Transmission shall be at adequate volume levels and free of excessive distortion. Levels of noise and cross-talk shall be such as not to impair communications. It is recognized that, due to extreme environmental conditions and economic factors, it is normal to have a small percentage of items that exceed objective levels. If the loss exceeds the stated objectives by more than 4 db, or the noise exceeds the stated objectives by more than 10 db, these cases of excess loss or noise shall be considered as troubles requiring correction.