If you run a business, any business, you plan for things like a lack of
a phone, or transport, or staff or anything otherwise you go bust.  It's
a form of natural selection and it works.

Any good business will not rely on just one form of communication.
Customers will be given a choice of alternative methods.  No company that is
properly registered with companies house will use just a mobile, they will
have a landline.  People often avoid companies using just a mobile,
especially if they have no alternative method of contact.  The mobile
companies do state in the contract that is read, understood and signed to
that effect by the customer that the service should not be relied upon for
certain things.  So using the excuses often given to claim compensation in a
slow month might not work.