Cómo se dice en inglés estar de guardia (un médico, un bombero, un juez)

On call

"She's a ​doctor, so she's often on call at the ​weekend.
Cambridge Dictionary Online.

"an on-call ​specialist/​firefighter/​judge"
Cambridge Dictionary Online.

In News:

"Hannah said while they were at the doctor's office, the nurse started
the ... Tennessee Valley OBGYN, was the doctor on call "
Times Daily