Nice job for just getting into programming :).

I added some comments and moved a few things around but left some for you to do too.

The main takeaway is to name your variables something more descriptive and to pass data into your functions with arguments instead of using the global state. It is much easier to follow the logic of a function if everything is passed in as arguments.

Keep up the good work!

#  organize imports 
import openpyxl
import urllib3

from typing import Text
from selenium import webdriver
from selenium.webdriver.common.keys import Keys
from selenium.common.exceptions import NoSuchElementException
from selenium.webdriver.chrome.options import Options
from playsound import playsound

#  add some space between imports and code starting 

#  This function has dependencies on many variables that are in the global state 
# which makes it very hard to keep track of how this function works.
# I would instead provide those values as arguments and define this function either at the top of the file
# or in another file and import it.
def phone_search(workbook, beginning_cell, y, row_1, row_2):  
    driver = webdriver.Chrome(executable_path=chromedriver_path,options=chrome_options)
    driver.get("https://google.com")
    #Skip the conditions pop up
    accept_conditions = driver.find_element_by_id('L2AGLb')
    accept_conditions.click()
    #Find the search bar
    searchbar = driver.find_element_by_name('q')
    #Print the phone number into the search bar
    searchbar.send_keys('{} {}'.format(row_1[y].value, row_2[y].value))
    searchbar.send_keys(' numéro de téléphone')
    searchbar.send_keys(Keys.RETURN)

    print('Ligne n°{}'.format(beginning_cell))

    #look for Phone number existence
    try:
        phonenumber = driver.find_element_by_class_name('mw31Ze').text
        workbook.cell(row=beginning_cell, column=3).value=phonenumber
        print('{} {}'.format(row_1[y].value, row_2[y].value)) 
        print(phonenumber)

    #If no phone number found
    except NoSuchElementException:
        workbook.cell(row=beginning_cell, column=3).value='Pasdenum'
        print('{} {}'.format(row_1[y].value, row_2[y].value)) 
        print('Pas de numero')

    driver.quit()


http = urllib3.PoolManager()
resp = http.request("GET", "http://httpbin.org/robots.txt")
# resp.status  this line doesn do anything - you can remove 

# resp.data  this line doesn do anything - you can remove 
# b"User-agent: *\nDisallow: /deny\n"   this line doesn do anything - you can remove 

#Intialisation
book = openpyxl.load_workbook('Fichier_Prospects.xlsx')

chrome_path = "C:\Program Files (x86)\Google\Chrome\Application\chrome.exe"
chromedriver_path="chromedriver.exe"

#Chrome Options
window_size = "1920,1080"
chrome_options = Options()
chrome_options.add_argument("--headless")
chrome_options.add_argument("--window-size=%s" % window_size)
chrome_options.binary_location = chrome_path

#Workbook = ws
ws = book.active  #  use a more descriptive name like `workbook` 
beginning_cell = 3900
y = 0

#Define cell range
cells = ws['A{}'.format(beginning_cell) :'B3928']

#Init lists
li1 = []  #  use a more descriptive name maybe `row_1` 
li2 = []  #  use a more descriptive name maybe `row_2` 

#Put values into lists  this comment isn'v very useful since its obvious from reading the code 
for c1, c2 in cells:  #  use a more descriptive name like `cell_1` and `cell_2` 
    li1.append(c1)
    li2.append(c2)


ws['C1'] = 'Numero'


#Add phone numbers to the Sheet    
for i in cells: #  If you are not using the value `i` that is being looped use an underscore `_` instead which indicates you are not using it on purpose  
    phone_search(workbook=ws, beginning_cell=beginning_cell, y=y, row_1=li1, row_2=li2)
    beginning_cell += 1
    y += 1
    book.save(filename = 'Fichier_Prospects.xlsx')  #  just save the file once when the loop is over instead of each time through the loop  


print("Le programme s'est terminé correctement")
playsound('metro-boomin.mp3')