Multiple myeloma (MM) is a plasma cell disease characterized by the growth of clonal plasma cells in the bone marrow. The purpose of this study is to assess the adverse events and change in disease activity of etentamig in combination with a cereblon E3 ligase modulatory drug (CELMoD) agent in adult participants with relapsed/refractory (R/R) multiple myeloma (MM). Adverse events and change in disease state will be assessed. Etentamig is an investigational drug being developed for the treatment of R/R MM. Study doctors put the participants in groups called treatment arms. Multiple doses of etentamig in combination with iberdomide will be explored. Each treatment arm receives a different dose of etentamig and iberdomide to determine a tolerable dose. Approximately 135 adult participants with R/R MM will be enrolled in the study in approximately 50 sites worldwide. In phase 1 participants will receive escalating intravenous (IV) etentamig in combination with oral iberdomide. In phase 2 participants will receive IV etentamig at one of two doses in combination with oral iberdomide, as part of the approximately 129 month study duration. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at an approved institution (hospital or clinic). The effect of the treatment will be frequently checked by medical assessments, blood tests, questionnaires and and monitoring of side effects.