There is a medical condition called Phonecall Anxiety
Some people actually develop anxiety, tremor, heart palpitation, jitteriness & uneasiness when their phone rings
For such people, try sending a message
They aren't rude for not picking. They're just protecting their health

Phone call anxiety can actually be so severe that if attached to some traumatic event in the person's past ..the person can be triggered and have some sort of PTSD
If you suffer from this The DO NOT DISTURB option on your phone can help
Consider seeing a Psychiatrist too.

So how it presents is whenever the phone rings some people can start having fast heartbeat.
Some people have even complained of disturbing headaches
And in some people there is a preemption if the phone will ring
It can really affect their day to day activities
Very disturbing

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