The main purpose of the study is to understand how safe and tolerable is elranatamab when given along with iberdomide. There are 2 parts to this study. Part 1 will look at how safe and tolerable is elranatamab when given with iberdomide. Part 2 will look at the correct amount of this combination that can be given to patients with relapsed or refractory multiple myeloma. Myeloma is a type of cancer that begins in plasma cells (white blood cells that produce antibodies). Refractory means a disease or condition that does not respond to treatment. Relapsed means the return of a disease after a period of improvement. All study medicines are given in cycles that last 28 days. Everyone taking part in this study will receive elranatamab as a shot under the skin. Iberdomide will be taken by mouth once a day for 21 days over a 28-day cycle. Participants will receive study medicine until: * their disease progresses or, * they experience unacceptable side effects or, * they choose to no longer take part in the study. The study will look at the experiences of people receiving the study medicines. This will help see if the study medicines are safe and can be used for multiple myeloma treatment.