When the full feature app will be out and smart phone users will be able to communicate with relays, users may not be willing to turn their phones into walkie-talkie. Imagine business meeting and your child calls you in! If the service is halted, they may miss on important messages.
I suggest that option to convert speech to text is implemented. Thus, phone users can opt to receive voice messages in the form of SMS. It could be setup for certain hours/days or turned on/off manually. Alternatively, conversion to SMS can be pushed at the Relay user level by voice prompt. Just like “OK Google”, it could be “OK Relay”.