Is it easy to crack a hashed phone number? I want to SHA256 hash phone numbers in order to hide them. Is this a good idea? Are there any other ways I could make this safe?
No, it is not a good idea to hash phone numbers. There are only a limited number of phone numbers, so it is pretty easy for an adversary to try and hash all of them. Then you can simply compare the hash of each with the stored hash. Generally you don't have to deal with all telephone numbers, only a subsection of phone numbers anyway (for a specific country or other group that is logically distinct).
You could use a slow password hash with salt and work factor, but that's only going to mean that time time required is multiplied by a large, constant value. It won't change the order of operations. If the subset is small enough it may not deter an adversary to perform all necessary calculations.
In this case you will probably need to encrypt the phone numbers instead. Or use a keyed hash such as HMAC. For both options you need to perform key management on the secret key though; it's not as easy as just hashing the number.