Hoping others will weigh in, but I wouldn't address this with the team member unless the team member has mentioned in the past that they prefer not to speak during client interactions. Everyone has a personal and different relationship with their stutter - some may welcome the opportunity to shine with an important client call while others would prefer to let someone else do the speaking. If they have looked for help during past conference calls then you may want to approach them, but otherwise I'd just recommend supporting them ("I know you'll do a great job on this call").