Registration is free, covers landline and mobile numbers, and lasts until you remove the number. It takes a few weeks to take full effect across the industry. A separate register, the CTPS, does the same job for business lines.
Being registered makes an unsolicited sales call unlawful unless you gave that particular company consent — and consent buried in a prize draw entry or a comparison site form still counts, which is why some perfectly legal calls continue afterwards.
The register binds companies that intend to follow the law. It does nothing about fraud, overseas call centres or recorded messages from operations that were never going to check a list; for those, blocking and reporting are the practical answers.
What to do: register the number, then report anyone who rings anyway to the ICO. Register only through the official TPS site — several paid sites charge for a free service, and some are collecting your details rather than registering anything.