Matrix

You can see Element's paid service offerings at https://element.io/pricing; their use cases for police is at https://element.io/sectors/interoperable-blue-light-emergency-services. Yes, it's a bit confusing to have both b2b and b2c under the same brand, but they do mean "the ability to encrypt communication within" and not your messages, similar to Slack's model. I'd also expect the community to catch any call-home attempt.