CREATE TABLE Students (cell_mobile_number VARCHAR, student_id VARCHAR)
CREATE TABLE Student_Addresses (student_id VARCHAR, monthly_rental VARCHAR)

What is the cell phone number of the student whose address has the lowest monthly rental?
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
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" NULLS FIRST LIMIT 1