ON call covering Physician

[macbook]

Hello,

 I am billing for a hospitalist A who was on call, and was covering for another physician hospitalist B to seeing patients in the hospital.

Do I bill like as a regular physician hospitalist A (on call covering physician) or do I append a q5 modifier? or is the hospitalist B supposed to be  billing for this and append the q5 modifier.

Please help.

Thank you,