To ask for someone on the telephone, use:

• May I speak to ... , please? (formal and polite)
• Is ... there? ( informal )

To leave a message, use:

• Can/Could/May I leave a message?
• Can you ask him/her to call me?

To ask someone to repeat something, use:

• Could/Would you repeat that, please? ( formal and polite )
• I'm sorry, I didn't catch that.
• Excuse me?
• Pardon?

Some alternatives to "yes" are:

• Of course.
• Sure.