The tests, due to start over the course of this week, will include trials of end-to-end encrypted audio and video calling.
At present, Secret Conversations only supports messages, pictures, video clips, voice recordings, and stickers being sent with end-to-end encryption, a protocol that is intended to prevent anyone other than participants from reading content, including platform providers.
Secret Conversations does not support encrypted group messages, payments, or audio/video calling, however, the social media giant has now begun testing extended encryption options for a potential rollout in the future.
Test group participants will see a phone icon at the top of the Secret Conversations window, as shown below, that can be selected to make a call. The option will be set in a similar layout to typical Messenger windows.
Facebook told ZDNet that the features will "give people more choice and controls" and that development in these areas is an "important step toward making Messenger a more secure and private experience."
The tests are expected to last several months. Potential rollouts may follow depending on the success of the trials.
In addition, the company is trying out a new timer feature. Secret Conversations already permits users to set a timer for their messages to expire, but the bolt-on will allow participants to turn off disappearing messages entirely -- or set a default timer for content to vanish based on one minute, 15 minutes, or 24-hour intervals.
The company has previously announced its plans to make chats across the platform encrypted by default, it's likely years before such a rollout is ready. In the meantime, the trials with Secret Conversations could pave the way forward in default encryption development.
"While we expect to make more progress on default end-to-end encryption for Messenger and Instagram Direct this year, it's a long-term project and we won't be fully end-to-end encrypted until sometime in 2022 at the earliest," the company said.