Hi (Name),
Sorry I missed you on the phone today, I was calling because…. (leave a one sentence reason for your call, or the name of the referral / event that introduced you)
In my voice mail, I mentioned that I will call you back on [DATE] at [TIME] and of course you can always reach me before then at [YOUR PHONE NUMBER].
I look forward to connecting.
Cheers,
[YOUR NAME]