。
If I go silent, contact the police, okay?
Literal
If radio-silence became-[conditional-たら] police [to-に] contact do-[request-てね].
音信不通 ('loss of contact, radio silence') — 音信 'correspondence/news' + 不通 'not getting through' — is a four-character compound for complete communication blackout. もし ('if') + なったら makes it hypothetical. The てね ending softens the request with feminine/gentle ね. The casual delivery contrasts with the alarming content.