mysql multiple unique values per primary key
You just need a composite (a.k.a. compound or multi-column) PRIMARY KEY:

CREATE TABLE callback
(
    f_unti_id varchar(10) NOT NULL,
    phone_number varchar(10) NOT NULL,

    -- Compound PK
    PRIMARY KEY(f_unti_id, phone_number)
);


(Actually, a UNIQUE constraint on multiple columns would suffice. Check How do I specify unique constraint for multiple columns in MySQL?).

Try it a http://sqlfiddle.com/#!9/9882a/1/0