This is from AAPC Coder:
"When reporting an "unlisted procedure code" to describe a procedure or service, it will be necessary to submit supporting documentation (e.g., the procedure report/OP notes) along with the claim to provide an adequate description of the nature, extent, need for the procedure, and the time, effort, and equipment necessary to provide the service. Also, submit a cover letter explaining why you have reported the unlisted code, instead of a definitive code.
Apart from reporting this unlisted code, mention one more code with similar type of service and bill the dollar amount associated with that code. The reason is that no fee is assigned to an unlisted code by the Medicare Physician Fee Schedule. But by comparing this unlisted procedure with another definitive CPT® code and its payment amount, you can explain to payers the reason for the amount that you have claimed. In your cover letter, explain the reasons for your billing the reimbursement amount. This can help a coder to avert a possible claim denial."