A vessel that reaches agreement with another vessel in a head-on, crossing, or overtaking situation, as for example, by using the radiotelephone as prescribed by the Vessel Bridge-to-Bridge Radiotelephone Act (
85 Stat. 164;
33 U.S.C.
1201 et seq.), is not obliged to sound the whistle signals prescribed by this rule, but may do so. If agreement is not reached, then whistle signals shall be exchanged in a timely manner and shall prevail.