The main purpose of this study is to learn about the safety of linvoseltamab and to find out what is the best dose of linvoseltamab to give to patients with multiple myeloma and to look for any signs that linvoseltamab can effectively treat cancer. The study is looking at several other research questions, including: * Side effects that may be experienced by people receiving linvoseltamab * How linvoseltamab works in the body * How much linvoseltamab is present in the blood * How linvoseltamab may work to treat cancer