Stubborn Partner – what can you do?

[Bluefoot]

I had a situation this weekend, NFHS scrimmage, when as BU, I had an OBS call, while my partner did not could not see me with my arm out - his back was to me. We discussed it afterward, and I learned that in that situation, when I know that partner can not see me give the OBS DDB signal, that I should vocalize the OBS call for him to hear. In the case posed for this thread, should the PU have vocalized OBS? Is vocalizing an OBS call written anywhere as a correct procedure for either ASA or NFHS umpires?