Telephone number gives error when minimum value is any number above zero

[Graniteman22]

I am not sure when it started happening, but when I make the telephone number mandatory (for account creation) by setting the minimum value above zero, I get the following error message

In the screenshot it says a minimum of 6 is needed, but I had actually input 10. If I turn the minimum value to zero, then even if I enter a 10 digit telephone number, it doesn't get passed through to the customer details. I have had two customers bring it to my attention, but have unsuccessfully been able to find a solution. I have been using Zen Cart for several years now and have never had this happen. I did upgrade to v153 in March but everything was working smoothly then.

Does anyone have an idea as to what I can do to fix this? Or what the problem could be? Thank you.

http://monumentsplus.com