Your obligation to your agency is to notify them when you have been released to work and to provide any physical restrictions through a completed CA-17.
The agencies and/or their nurses are inserting themselves into claims and asking providers for information such as billing codes. Legitimate OWCP Field Nurses don’t need to ask for billing codes. Since all your bills go through OWCP, they can easily find out what’s being billed. They don’t need ask your providers for billing codes.
If I haven't said it enough: The agency should contact OWCP if they need information about your claim.