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A fighting dog is hard to call back.
Literal
Mutually-bite dogs [topic-は] hard-to-call.
An archaic/classical-flavored proverb. 噛み合う ('mutually bite,' 'fight [of dogs]') uses the reciprocal ~合う ('do X mutually,' 'do to each other') attached to 噛む. The reciprocal ~合う is productive: 話し合う (talk with each other), 愛し合う (love each other), 助け合う (help each other). 呼び難し uses the classical ~難し (がたし), the predicative form of 難い (がたい, 'hard to'), surviving today in set proverbs and poetic writing. The modern equivalent is 呼びにくい or 呼ぶのが難しい. The whole proverb is in a literary register: the subject-predicate structure without polite endings, and the classical 難し, mark it as a saying rather than everyday speech.