POST /api/v2/conversations/calls 400 bad request participant address

Struggling to figure out why my grpc service gets 400 when calling POST /api/v2/conversations/calls. the payload looks fine but the error says malformed participant address.

{ "from": { "phoneNumber": "+6512345678" }, "to": [{ "phoneNumber": "+6598765432" }] }

is there a specific format i am missing? it works in postman but fails in my code.