GXW4108 not auto hang -up pstn line after transfer call


I use Panasonic TDE600 and connect 2 analog line to GXW4108 and use WP820 as wifi phone. When we receive a call from Panasonic, WP820 will ring. Everything 's ok except transfer function. When we transfer a call from WP820 to some extension on PABX, it can transfer well but the line on GXW4108 not hang up after call end. What i need to do on GXW4108 to hang up line when a transfer call is end


From WP820 is hang up, but from GXW4108 show busy