When a communication is sent, the communications service needs certain metadata in order to operate. This includes user identifiers and timings (in other words, the 'who', 'where', 'when' and 'how' of the communication). This imparts information about users of the system, especially in aggregate, and if misused it could reveal the individual connections of users of the system.
You should ensure that such metadata is only used for the necessary operation of the service. In particular, that it is not harvested, sold or exploited by the communications service. The communications service should provide clear and transparent terms and conditions that set out what content and what metadata is collected and processed, and for what purpose. Moreover, you should have confidence that these will be followed and are appropriate for your corporate need, as opposed to a consumer need.