A student is struggling to answer a question so you allow him or her to contact the expert (The instructor). This strategy needs guidelines, or the expert will be contacted every time. Consider the following suggestions should you choose to use this strategy.
Allow this strategy to used only once during the lesson (or however many times you choose). Once applied it can not be used by anyone else. Tell students they need to decide when to use this strategy.
When they do use the expert give them a choice of two or three answers. They need to choose the right one and then EXPLAIN why that answer is correct.
The burden on this strategy is the instructor needs to be prepared to have various answers available. Depending on the question the answers all can be correct, and the burden falls on the student to pick one and explain why. Or there is only one correct answer, but again right or wrong the student needs to provide the reason they choose the answer given by the expert.
Another option is to have the instructor give an answer - right or wrong and the student needs to explain whether they agree or not with the expert, and WHY.