I haven't personally billed for a tens unit but was aware of the AMA CPT Assistant article below:
Question
Should I use the modality codes from the medicine section of the CPT book to report application of a TENS unit?
AMA Comment
CPT code 64550, Application of surface (transcutaneous) neurostimulator, is intended to report the initial application of a transcutaneous electrical nerve stimulation (TENS) unit in which electrodes are placed on the skin by the physician and the patient then takes the unit home. The patient operates the TENS unit at home (eg, changing voltages). This code identifies the initial application before the patient leaves the office.
CPT codes 97014, Application of a modality to one or more areas; electrical stimulation (unattended), and 97032, Application of a modality to one or more areas; electrical stimulation (manual), each 15 minutes, refer to physical therapy modalities, where the electrical stimulation is applied in the physical therapy department; either applied where the physician/therapist does not need not to be in constant attendance, or where the physician/therapist needs to be there to constantly supervise the application.