How to customize entry widget and save input with tkinter

You don't need an additional function to save the extra input, just to modify getInput().

What you can do is define an extra StringVar, phone_var, for the phone number (it will be '' by default) and connect it to the extra entry you create in addEntry(). Then, in getInput() you can get the content of phone_var (which always exists even if the entry doesn't). If it is '' then you don't save it.

import tkinter as tk

root = tk.Tk()
root.geometry("600x400")

name_var = tk.StringVar()
surname_var = tk.StringVar()
phone_var = tk.StringVar()  # extra StringVar for the optional entry


def getInput():
    name = name_var.get()
    surname = surname_var.get()
    phone = phone_var.get()

    print("The name is : " + name)
    print("The Surname is : " + surname)
    if phone:  # save it only if not empty
        print("The Phone is : " + phone)

    name_var.set("")
    surname_var.set("")

def addEntry():
    phone_label = tk.Label(root, text='Phone', font=('calibre', 10, 'bold'))
    phone_entry = tk.Entry(root, textvariable=phone_var, font=('calibre', 10, 'normal'))
    phone_label.grid(row=3, column=0)
    phone_entry.grid(row=3, column=1)


name_label = tk.Label(root, text='Name', font=('calibre', 10, 'bold'))
name_entry = tk.Entry(root, textvariable=name_var, font=('calibre', 10, 'normal'))

surname_label = tk.Label(root, text='Surname', font=('calibre', 10, 'bold'))
surname_entry = tk.Entry(root, textvariable=surname_var, font=('calibre', 10, 'normal'), show='*')

name_label.grid(row=0, column=0)
name_entry.grid(row=0, column=1)
surname_label.grid(row=1, column=0)
surname_entry.grid(row=1, column=1)

save_btn = tk.Button(root, text='Save', command=getInput)
save_btn.grid(row=5, column=0)

add_btn = tk.Button(root, text='Add Info', command=addEntry)
add_btn.grid(row=5, column=1)

root.mainloop()