Problems with customers not filling required fields and yet able to register

I have a recurring problem, some customers don't enter their telephone or other required fields, and somehow they get registered, so I think it has to do with validation, because these fields are required, and they simply skip them! How that can be fixed? Is there a bug in the validation process after the form gets submitted? I never looked at the script that controls that, so I am clueless. I am talking about the register form.

In most cases I've seen, customer registrations which don't include the information required on your site have come from Paypal, when the customer enters his/her details on their site. Could that be the case with you ?

stevesh:

In most cases I've seen, customer registrations which don't include the information required on your site have come from Paypal, when the customer enters his/her details on their site. Could that be the case with you ?

Hmmm, normally they come from Google Shopping, didn't know Paypal sends traffic ... BTW
They use Paypal to checkout afterwards, because that's the only way I allow them to pay with their credit cards ...

In my logs I couldn't find any reference to Paypal.com ... as the referring domain ...

What I meant was that if visitors use the Paypal Express button to go to Paypal to pay without creating an account on your store, Paypal returns the address, etc. information back to Zencart.

ok, In some cases they might, but my question is, about customers that come register and use a regular Paypal checkout, not Express, I thought it might be a bug or something ... Does the Paypal script insert the customer data into the database?