Dear {Name},
I can't apologize enough for not returning your phone call on {date}. I hope my lack of response hasn't been a huge inconvenience. I'm now working on getting you the {information/documents/contacts} you requested and will call you as soon as I have that ready. I'll also send you the information via email.
In recent years, I've shifted away from voice calls and now rely more on email and text messages. You can reach me anytime at {email address} or {text messaging number} and be assured of a quick response.
But that's no excuse for not returning your call. I did receive it, and my support staff and I are working on a better system to make sure that important messages like yours don't slip through the cracks again.
Sincerely,
{Name}
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