French phrase
Veuillez ~ si nécessaire.
Meaning
Please ___ if necessary.
When to use it
A polite request to act when needed. Formal and common in written instructions or service messages.
Example dialogue
Si j'ai des questions, qui dois-je appeler ?
If I have any questions, who should I contact?
Veuillez me contacter si nécessaire.
Please contact me if necessary.