。
In case of emergency, call this number.
Literal
By-chance's case [topic-は] here [to-へ] telephone [object-を] give-[please-ください].
万一の場合 ('in case of (the) unlikely event' = 'in an emergency') is a set phrase combining 万一 with 場合. へ marks the direction of the call — 'call to here.' ください attached directly to a noun (電話をください 'give me a call') is a polite request for the noun's equivalent action.