SELECT T2.cell_mobile_number FROM Student_Addresses AS T1 JOIN Students AS T2 ON T1.student_id = T2.student_id ORDER BY T1.monthly_rental LIMIT 1
What is the cell phone number of the student whose address has the lowest monthly rental?
CREATE TABLE Students (
    cell_mobile_number VARCHAR,
    student_id VARCHAR
)

CREATE TABLE Student_Addresses (
    student_id VARCHAR,
    monthly_rental VARCHAR
)