Hi Bernadette, these are all very good questions. The ABN is to be used if there is presumption of non coverage of an item or service that is normally covered. For example, if you dispense a Cam boot and you know the patient had one previously or suspect they had a "same/similar" item, you would have them sign an ABN. In the case of "routine foot care", technically speaking, it is not covered by Medicare and thus does not require an ABN. With that said, the patient needs to be notified that their treatment is not covered prior to treatment. The only reason you would use the G modifiers is if there is a "secondary" insurance (not supplement--they only pay what Medicare covers) that might pay. In that case you would add the GY modifier. In most cases, since most patients have a "medicare supplement" plan, you would not send a claim, the treatment would be CASH pay at the time of service.
I should add that some of the Medicare jurisdictions pay for callus and nail treatment if the provider can document that the patient has pain. Noridian for example pays for 11721 and 1105x with the secondary diagnosis M79.67_. I hope this helps.