On calls form part of most NHS doctor jobs. The pattern of on calls varies from job to job, but can include long days (until 7-10pm at night), twilight shifts (until 1-2am), night shifts and weekends. The good news is that you are paid equivalent to the number of on calls required by your rota.
During the week daytime hours (8am-5pm Monday-Friday), you will usually be part of a full team, led by a consultant. The team is allocated a set of patients (usually around 20). Out of hours (between 5pm and 8am during the week, or any weekend/bank holiday), you may be on “ward cover”. During these shifts you are part of a smaller team with whom you may not have worked before, and this team are responsible for a larger number of patients (commonly over 100) across several wards. You may need to cross-cover another specialty e.g. if you are an acute medicine doctor, you may cross-cover stroke, cardiology and gastroenterology overnight. You do not perform routine day-to-day tasks; your duty is to respond to nurse bleeps to review unwell patients or perform urgent tasks. During weekend days you may need to do a ward round of unwell patients as requested by the patient's usual team.
Alternatively you may be working as part of the medical “take” team where you clerk new admissions to hospital.
The on calls might sound intimidating, but there should be more senior doctors available at all times to answer clinical (or non-clinical) questions and offer you support when it is required. The on call shifts are generally busy, but time passes quickly and you gain a lot of clinical experience.
When applying for a job it is a good idea to find out what the on call duties involve, if you will be covering different specialties, and the structure of the out of hours team.