[Plugin: Contact Form 7] Telephone number in optional field flagged as spam

I recently discovered that a form that I had created was not working properly. I had thoroughly tested it when I first created it, but sometime in the last few weeks, I guess, it has started flagging submitted forms as spam. It had two required fields followed by an optional telephone number field as below:

Your Name (required)
    [text* your-name akismet:author] 

Your Email (required)
    [email* your-email akismet:author_email] 

Telephone (optional; please include area code)
    [text telephone 15/] 

Best time to call?
    [radio besttime "Morning" "Afternoon" "Evening"] 

Reason for contacting Nature by the Yard?
    [select* contactreason include_blank "Register for a Program on the Calendar" "Schedule a new Nature By the Yard program" "Get more info about programs or fees" "Inquire about the Nature Inside Out book" "Other"] 

Additional questions or comments?
    [textarea your-message] 

[submit "Send"]

Somebody alerted me to a problem, and I discovered that whenever text (numbers) was placed in the telephone number field, the message would not be sent due to “# Akismet judged the sending activity as spamming”. This same behavior occurred regardless of the name and email address used in the preceeding two fields. If I left the telephone number field blank, the message would send just fine.

As a workaround I created a new form without the telephone number field and instructed visitors to add a telephone number with comments in the textarea field. Unfortunately, a string of digits like a telephone number in this field also causes it to be flagged as spam (orange border)!

Any ideas what went wrong, because I’m sure this worked when I first created it about 6 months ago? Neither of these fields should be subject to spam filtering, so why is it getting rejected?