No wireshark trace with SDP changes


I’m using Asterisk/
I have 2 SIP Phones (extensions 6001 and 6002) that are connected

When 6002 puts the call on hold, wireshark displays the new INVITE with SDP set on ‘sendonly’’ at the senders end. So far so good.
As to be expected the receiver (6001) is put on hold (onhold-music is played) but a wireshark trace at the receivers end displays no data whatsoever.
No SIP messages are received??? None!!

Only with initial INVITEs and BYE, messages are received and can thus be traced.

How can I have changes in OnHold status be received as well?

thank you
ps: no problem when using Innovaphone for instance, changes in status are detectable at the receivers end

Answered on, more appropriate, Asterisk Support.