Hi Carl,
no the trust boundary stays on you switch, which is the network boundary where you take the decision whether trust QoS marking received from 'external' devices or reset them.
I understand that your doubt might arise from the fact that you also own/administer the ip phones, but the trust boundary is a concept typical of switched/routed networks and apply to devices which switch/route traffic only, not to the ones which generate such traffic (phones, pcs, servers or all other hosts in general).
Riccardo
