In general, any time you use an unlisted code you should be including a description of the service, whether it is on the claim form or in the medical records. If you are providing the description in the medical records, that should suffice but from an insurance company's perspective it is helpful to have a description of the procedure on the claim form in addition in the medical records.
With respect to your desire to bill 97799-Unlisted physical medicine/rehabilitation service or procedure instead of the existing/defined CPT codes for OT and ST because the reimbursement is too low on those existing codes, this is not appropriate coding. Changing codes for the purpose of increasing reimbursement is a quick way to get your provider on a list of providers to be audited by the payers you are submitting claims to.
Additionally, your OT & ST providers are unlikely to be providing services that qualify for billing E&M codes instead of the appropriate physical medicine/rehabilitation codes and again would not be appropriate if they are not rendering E&M services as defined by CPT. Additionally, most plans have benefit limits for rehabilitation or habilitation OT & ST services and the benefit for these services might be different than the benefits for an office visit E&M.
Just because you are getting other insurance companies aside from Aetna reimbursing you for 97799, don't be surprised if the other companies come back to you in the future after auditing your provider's claims and requesting any overpayments back because of incorrect coding.
Were you directed by your compliance specialist or management about making these coding changes just to receive higher reimbursement than you currently do because if you make this decision on your own you could be liable for any repercussions for knowingly submitting claims with incorrect codes for the purposes of receiving better reimbursement rates. I would tread very lightly in changing how you code claims because someone has decided the existing reimbursement rates being too low for the appropriate and existing CPT codes for OT & ST services, you don't want to be the one left holding the bag and being liable for these decisions if you did not make the decision to use alternative codes to receive better reimbursement rates.