Currently, we are developing an ACS-to-ACS call solution for one of our customers, and while the call placement is working perfectly fine, we have encountered an issue when trying to add a PSTN number to an existing ACS-to-ACS call. Specifically, we are faced with error code 400. Our solution utilizes the Frontend SDK (@azure/communication-calling) to create a peer-to-peer call. Could you kindly provide your insights on how to resolve this issue? Your assistance would be greatly appreciated.