If canreinvite is set for both ITSP and tie trunks (and sys2 isn’t doing anything to preclude reinvite such as recording or listening for DTMF), audio should be automatically redirected when the call is answered. And with canreinvite off for sys2 extensions, those calls will have audio relayed normally.
However, most trunking providers allow you to configure inbound routing on a per DID basis. Check your provider portal for this option.