Yealink w80: handsets do not send 100 trying

[bjorn.peeters]

Hardware:
1x w80DM (103.83.0.80)
3x w80B
7x W53 handset

Situation:
out of the 7 handsets/accounts, several do not send back a 100 trying on an invite request.
so they start ringing, but do not confirm to the sip server they are ringing.

this causes them to continue ringing (and even allowing for picking up a non-existent call) when it has been answered elsewhere.
it also causes them to be able to pick up a call without actually picking up (the phone will display connected, but the call will not have been picked up since there has been no confirmation sent to the server)

there is no problem on the netwerk, nat, or server (since some do it correctly and ALL fixed yealink devices work fine)

is there a setting that can be changed to prevent this, or is this an actual bug which has not yet been reported/fixed?