Rheumatoid Arthritis Treatment

I know a medicare advantage plan requires me to pay a 20% co-pay for drugs covered under part b medicines. But I understand that if I have basic medicare along with a medigap policy (either plan f or J), it will take care of all the part b meds. Do you know if i will need to pay a co-pay? I asked the medical billing department at me doctor’s office but she was really hesitant to tell me. Oh, in case it matters: it is Orencia — infusions for rheumatoid arthritis — my doctor’s office does the infusions out of his office so that it does fall under part b drugs.