It’s not just phone calls but other forms of modern communication such as the example you mention which involve speech, and being able to hear that of others. But the same can apply to the traditional face to face conversation, it’s the etiquette and not whatever technology that’s the issue.
On the other threads some have advised that asking the offenders politely to refrain or consider others is the answer. Both personal experience and that of witnessing others is that just seems to aggravate the perpetrator(s) – maybe I’ve just been unlucky and missed those who are able to reflect on their behaviour having been approached.
Someone told me once that the best tactic is to appear to benefit from overhearing the confidential information that is being broadcast – ok not much use in the example of the family gathering. Once the offender’s phone call has ended, pretend to make one yourself to your CEO divulging all the free IP that you have just gained. Although equally verbose, others who were fed up with the original will soon appreciate the prank!
For a more silent protest in keeping with the wishes of all, tactically brandish your fake GCHQ security pass – it’s free with every Westminster one.