400 Bad Request for Telephone numbers

Hi there, using this plugin for the first time. Integrating Contact form 7 with Klaviyo.

When I fill out the form and submit, it does not add the entry to Klaviyo. When I check the log file it says the following:
{“detail”:”+07********* is not a valid phone number.”}

Can you help assist with the issue please.

Thanks in advance,
Kev