Secure Anonymous Messaging - Seeking Recommendations

[JustPlayItLoud]

I'm trying to find a messaging/SMS/chat app. In addition to standard anonymity and encryption I'm also trying to find something with the following:

1. Not tied to a phone number
2. Something at least similar to the following: Ideally you would be able to create a conversation that can only be joined via password given out to invited persons. Even better if the "conversation" expires after a predefined time and a new one must be started. The ability to designate your own nickname woudl be icing on the cake, but not necessary.
3. Android and iOS support are essential.
4. Needs to be free.

I've looked at recommendations for secure/encrypted messaging apps but no descriptions I've found indicate of they have any of the features I'm looking for and I figured this might be easier than testing them out one at a time.

For reference, this is not going to be used for anything illegal or classified. Occasionally we have need of discussing work related items that the administration "officially" does not allow despite it being a virtual necessity.

Any discussion medium using work computers can obviously be tracked by IT, and we need something that cannot be tied back to any identifiable information (phone number, email) in the event that the communications are compromised.

There is a virtual 100% chance I'm just being overly cautious, but I'm going to do it anyway.

Any recommendations or suggestions are most appreciated and will make you worthy of all the interwebs cookies.

[BishFish]

I haven't tried this, but the Signal website says you can create an account with a google voice number. Could you all create google voice number from a new gmail account? Getting the Signal app on your phones while still using the google voice number, may not be possible, but I haven't tried it. Signal has all the other features you want - encrypted, expiring messages, and free.

[xxVEG]

An acquaintance of main mentioned an app to me a while back that I think was called ghostdust or cyberdust or something. Its messages are deleted and it's supposedly pretty dang secure