It's easy to imagine the caller hanging up, dialing another prospect, and leaving an identical voicemail using the exact same high pitch, and then another … and another. If it sounds like a salesperson is just doing their 50 prospecting calls for the day, it absolves the listener of any responsibility to respond.
Or, if they need to refinance, tell them that you know about current rates. And, you can provide them with guidance on if the timing is right. Staying honest and genuine will give you credibility and encourage them to return your call. After all, you are trying to become their trusted advisor, and the voicemail is the first attempt at this.

Nobody wants to play phone tag. Skip the back and forth by explicitly telling the caller to leave their name, number, and the best day(s) and time(s) to reach them. Your clients will appreciate you not wanting to waste their time.
Any other thoughts? Write your voicemail for optimal message, and socialize the voice mail with your manager or your peers. Take the time to read it, time it and record it until you feel comfortable with your tone of voice, inflexion and energy. Write three or four voicemails with different scenarios and value statements; remember that you are trying to create an impression and recognition of your name and your solution.