I have missed, and promptly returned, two calls from the HR recruiter. One to schedule a phone interview, and the second to schedule an in-person interview. 

Both return calls were about 20 minutes or so from the initial missed call, and I apologized both times. However, these were brisk apologies, and I didn't think to provide a reason for missing the calls. I just kindly said "I am sorry for missing your call" and went on with scheduling the interview.

I feel as though I should send an email to the recruiter and further acknowledge my apologies for missing both calls. I don't want the recruiter to think that I don't respect their time. 

Would an apology email be too much? I am expecting an interview confirmation email, and this could be a natural response.