Hello ppl. I ll try to keep this question as simple as possible! 
What is the best way to make Customer entity details such as phone or email required? The user should complete those according to mask before proceeding to save them. I have spent some time looking for it in the forum without any luck.
Whichever one is set as Primary Field Name will be required.
and in case I want to have more than one required fields?
That is not a feature that is currently supported.
You could create a custom [?prompt] sequence to create the entity and in doing that way you could have the fields required.
Or use a script.
That means he would need to design his own customer Entity screen for entering customers.
Yes, like my booking entity setup. Didnt say it was ideal easy solution 
hah okay thank you very much both of you ! I ll start working on that custom Entity screen then!
Search for RegEx expressions in Custom Fields. You can make fields mandatory that way…