Give the phone number of the only student who obtained "A" in the course "Intro to BlockChain".
A refers to an excellent grade in which grade = 'A' for the course;
SELECT T1.phone_number FROM student AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id INNER JOIN course AS T3 ON T2.course_id = T3.course_id WHERE T3.name = 'Intro to BlockChain' AND T2.grade = 'A'