Here a link to the manual.
Panasonic KX-T7716C Quick Reference ManualIt is explained that it also requires batteries, which leaves me scratching my head.
It does explain this is indeed to be used with a PBX. This appears to be compatible with a Panasonic KX-TA824. It will probably operate with the KX-TA624 too.
The two center wires of the telset's cord are wired for ring and tip. This is why you may have dial tone upon lifting the receiver. I doubt though that all of the other features will work.
<Seems I spoke too soon, no wonder it left me scratching my head>