Is it illegal to send a text message to a new number?

Unless you are harassed every day, it is not illegal.

Generally, threatening others by sending text messages will not constitute a crime, but will only be punished by public security, and the police generally will not file a case. However, if you threaten others by sending short messages many times, it will have a bad social impact; Threatening to cause other bad circumstances will constitute a crime, and the police will file a case at this time.