First of all you should call your lawyer and ask him what your legal options are. In the USA Federal law requires courts to make reasonable accommodation for ones religious beliefs, and delaying a phone call a day or two should be considered "reasonable accommodation". Also, you should contact Agudath Israel or the OU, as these and similar organizations have extensive experience dealing with situations where demands of the courts conflict with religion. However it may be difficult to reach them on such short notice on Sunday erev Yom Tov.
If all else fails, you may ask a non-Jew to place the call on your behalf on Yom Tov, hopefully this will satisfy the Court's demands.