Simply posting a phone number and inviting people to call it may be classed as harrassment. If the phone number is accompanied with identifying information like name, address, birth date etc., then this is also a breach of personal data.
Both are illegal. However, finding out who did it and then prosecuting them is another matter.
Police are unlikely to act and even if they did, it would not be a priority given their overstretched resources as it is.
A civil case for damages could be sought, but this is a costly route fraught with traps to foil claims.