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You should avoid phoning people after 10 at night.
Literal
Night 10-o'clock after [at-に] person [obj-に] phone-do-[for-には] avoid-[should-べきだ].
~以後 ('after, hereafter') is the formal marker for 'after a certain time.' Contrasts with ~以降 (also 'after, from X onward'). ~には purposive again: 'for the purpose of (doing X),' here with a negative consequence — 'should avoid doing X (for phoning purposes).'