Hi Hani,
You can add this below piece of code in your detail_in.htmpl file.
<PDM_MACRO name=dtlReadonly hdr="Contact Number#" attr=customer.phone_number>
Note: This is just a read-only field.
You wont be able to update this field and even if you manage to create a new column and update the value of the customer phone, you cannot directly update from incident detail page.
You might want to consider some Spell code / triggers that would take care of this update in ca_contact table.