You can check the Configuring Supervisory Disconnect section of the following link
http://www.cisco.com/c/en/us/td/docs/ios/voice/fxs/configuration/guide/15_1/fxs_15_1_cg_book/fxsbasic.html#wp1019397
If it was working fine earlier then please share the running config and IOS version of the VG224.
Has a reboot / reload of the VG224 been tried already?
HTH
Manish