I see internal out links to 4xx on product pages. see this URL 
view-source:https://classifiedsgulf.com/kolkata-services/information-technology1-classified-clone-script_i18931
 and search <a href="No phone number
please help us understand where it comes from and how to fix it.

You probably need to add check and in case there is no phone number, out there # instead of text.
But generally it probably does not matter at all