The locum tenens rules are Medicare rules that allow for a physician practice to bill for a substitute, temporary physician under the provider number of a physician who has left the practice.  

• The physician must be absent from the practice due to illness, vacation, CME or the physician may have left the practice
• The regular physician must be unavailable to provide the service
• This is a physician rule and may not be used for Non-Physician Practitioners (NPPs)
• There is a limit of 60 days for each locum physician, counted from the first day the locum sees a patient.  This 60 days includes days that that locum does not see patients (the locum's day off)
• After 60 days, you may no longer bill for that locum.  If services are still needed after that, the practice must find a different locum
• Bill for the locum under the NPI of the physician who is not there, and use the Q6 modifier on the claim
• You must keep a list of patients seen by the locum
• The regular physician pays the locum on a per diem, or a fee-for-time basis
• If post op care is rendered by the locum in the global period does not need to be identified on the claim form
• This may not be used when you are adding a physician and you have not enrolled the physician.  Review the enrollment rules.

There was an exception for the first half of 2008 which allowed physicians who were deployed on active duty  in the Armed Forces to use a locum for longer than 60 days.

This is a Medicare rule.  Commercial insurances generally do not allow this type of billing.