atcomsystems.ca/forum Forums TDM Business Telephone Systems Panasonic Telephones & Systems KX-TA824 and KX-TVA50 APT connection

The TVA 50 supports 4 APT ports, 6 DPIT, and 6 Inband.
You say it works with inband. If working with inband then it is programmed that way.
If both are set for apt it would work, unless 1 of the units is bad.
You need to contact where you bought the equipment from or call in a qualified tech to check system out.

using 4 conductor line cords?
install manual 1,2,2 connection diagram.