Presumably you are using standard BT analog lines ?

If so, these do provide a end-of-call signal but it is fairly short - between 90 and 130 mSec as detailed in SIN351 section 7 <https://www.sinet.bt.com/351v4p5.pdf>

I believe you can ask BT to increase this, but trying to navigate their customer support to someone who knows about this can be difficult, so in the past I've reduced the CO SUPV TIME in TRUNK-WIDE TIMERS as the default is 400 mSec so the 816 fails to see the signal.