SQL Server 2019

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CREATE TABLE PersonsInitial (
    tel int,
    firstname varchar(255),
    lastname varchar(255),
    Customer_ID varchar(255)
);
insert into PersonsInitial(tel,firstname,lastname,Customer_ID) values
('01234','Manu','Johns','456'),
('01234','Manu','Johns','C456'),
('21234','Fernand','Wajk','C389'),
('13554','Mathieu','Wainers','4683');

select distinct tel, firstname, lastname, customer_id from PersonsInitial

--if there is a person with the same tel number chose the customer id with 'C'
--if I don't have the choice add the customer without C

CREATE TABLE PersonsFinal (
    tel int,
    firstname varchar(255),
    lastname varchar(255),
    Customer_ID varchar(255)
);

insert into PersonsFinal(tel,firstname,lastname,Customer_ID) 
select distinct pi1.tel, pi1.firstname, pi1.lastname, coalesce(pi.Customer_ID, pi1.Customer_ID) Customer_Id 
from PersonsInitial pi1
outer apply (select * 
     from PersonsInitial pi2 
     where pi1.tel = pi2.tel 
         and pi1.firstname = pi2.firstname 
         and pi1.lastname = pi2.lastname
         and pi2.Customer_ID like 'C%') pi;

select * from PersonsFinal

tel	firstname	lastname	customer_id
1234	Manu	Johns	456
1234	Manu	Johns	C456
13554	Mathieu	Wainers	4683
21234	Fernand	Wajk	C389

tel	firstname	lastname	Customer_ID
1234	Manu	Johns	C456
13554	Mathieu	Wainers	4683
21234	Fernand	Wajk	C389