A polite way to request to call a client is to send a respectful and professional message. Here's an example of how you can do it:
Subject: Request for a Phone Call
Dear [Client's Name],
I hope this message finds you well. I would like to discuss [briefly mention the topic or purpose of the call] with you in more detail. As I believe a conversation would be more productive for addressing [any specific concerns or issues], I would be grateful if we could schedule a convenient time for a phone call.
Please let me know your availability, and I will ensure to adjust to your preferred date and time. If there's a specific number you'd like me to reach you at, please provide it for my reference.
Thank you for your understanding, and I look forward to speaking with you soon.
Best regards, [Your Name] [Your Title/Position] [Your Contact Information]
Keep in mind that the key to a successful business interaction is being professional, clear, and accommodating to the client's schedule. Always show appreciation for their time and consider their preferences when suggesting a call.