Your in-house coders create their own Instant Messaging application.
All messages are encrypted in transit and flow through centralized company owned servers where any and all messages can be retained for however long you need them to be.
Ours supports simple text messaging, behind the scenes encryption, file transfers, multi-user group meetings, screen sharing / remote access, employee searches, etc. etc. It's probably more robust than many commercial texting systems are.
Hell, I even get a daily digest of all my messages in an email so I can keep them for record purposes and future referencing myself.