fwiw I did manage to get this going eventually, just posting solution here for future reference. there is some magic required in the Contact header to include the tgrp parameter (generally your pilot number - 91234567 as an example)

something like this

voice class sip-profiles 100
��rule 110 request INVITE sip-header Contact modify "@" ";tgrp=91234567;trunk-context=spark.co.nz@"

cheers,

Blair