You shouldn't make phone calls at this hour of the night.
Literal
Night [gen-の] such time [at-に] phone [obj-を] make-[general norm-ものではない].
~ものではない ('one doesn't do X, one shouldn't X') is the prohibition version of ~ものだ — expressing that something is not done as a general norm. こんな時間に ('at this (kind of) time') implies 'at this ungodly hour.' 電話をかける ('make a phone call,' lit. 'hang a phone').