No more than one page. Condense. Include only what is most important.
Address the responsible HR officer personally (not “Dear Sir or Madam” but rather “Dear Ms. Ernoult”, for example).
Write one or two concise subject lines, no more than that. (You will find further hints and examples in the documents linked below.)
Write a concise introduction, as well. HR officers have limited time. Start with something that will catch their interest. But do not try to be too “witty”. Avoid standardized phrases, but remain business-like. Try to be authentic but serious.
Highlight your specific qualifications, clearly relating them to the vacant position. What makes you particularly suitable for the post? Again: No standardized phrases.
Refer clearly to your telephone call. Not boilerplate, but rather tailored.
Mention a specific time slot. (If not provided, when is the best time for you?)
Don’t forget the recipient’s address. Don’t forget the sender’s address including phone numbers. You want them to call you back, don’t you?