Add an AND condition only IF a certain expression is true - mysql

Question

I want to add an AND condition only if an expression is true, otherwise the AND condition should be ignored.
I'm supposed to group concat the phone numbers if they have IsConcat set to true, otherwise I need to select the first one which doesn't have IsConcat set to true.
What I've tried so far:
SELECT IF(p.IsConcat = 1, GROUP_CONCAT(p.PhoneNumber), p.PhoneNumber) AS 'PhoneNumber'
FROM phones p
WHERE p.IdStudent = _idStudent
AND IF(p.IsConcat = 1, TRUE, '') -- only check if IsConcat = 1 otherwise i don't need the AND statement
LIMIT 1
I saw somewhere online that TRUE is being used in if statements, thought it would work but it didn't.
I have some mock data: Two students, the student with Id = 1 has 5 phones and only 3 of them have IsConcat = 1 -> the output works good since it does a group concat and shows only those 3 numbers.
The student with Id = 2 has two phone numbers and both have IsConcat = 0, which means I need to select the first one, without checking if IsConcat = 1, that's why I added LIMIT 1. In that case it outputs NULL.
The expected output if the phones table has any numbers for the appropriate student with IsConcat = 1 should group_concat only them.
If the phones table has no phone numbers with IsConcat = 1 for the appropriate student, it should output the first phone number.

Answer 1

I found a solution, thank you #Akina for giving me the idea to use SUM().
If SUM(p.IsConcat) > 1, it goes to a subquery and does group_concat to ones that have IsConcat = 1.
This is the final solution
SELECT IF(SUM(p.IsConcat) > 0, (SELECT GROUP_CONCAT(pp.PhoneNumber)
FROM phones pp
WHERE pp.IdStudent = _idStudent
AND pp.IsConcat = 1
LIMIT 1
), p.PhoneNumber)
FROM phones p
WHERE p.IdStudent = _idStudent
LIMIT 1