0120 numbers are toll-free numbers on which the receiver pays the call charges. The caller pays nothing, and the company that receives the call bears the cost. The number is 10 digits in total: 0120 plus six more (e.g., 0120-XXX-XXX). "Free Dial" is a registered trademark of NTT Communications, and other carriers offer 0120 numbers under service names such as "Free Call."
0120 numbers are everywhere on corporate contact lines: customer support, order desks, brochure requests and complaint handling. Because the caller is not charged, the number lowers the barrier to getting in touch, which is why television commercials announce "Call 0120-XXX-XXX."
One thing to watch for is that a 0120 number may not accept calls from mobile phones. Calls from mobiles cost the receiving side more, so some companies choose not to accept them, and callers hear a recorded message saying the number cannot be reached. When that happens, look on the company website for an ordinary phone number (one starting with 03, 06 and so on) or for whichever alternative desk number is listed. If the number you are directed to is a 0570 number (Navi Dial), the caller pays for the call and it is not covered by mobile flat-rate calling plans, so keep an eye on how long you talk.
0800 numbers are a similar service. They are also receiver-paid, and they were introduced because 0120 numbers were running short. The six-digit portion of 0120 allows only one million combinations, and the Ministry of Internal Affairs and Communications listing of assigned numbers shows roughly 990,000 of them already allocated as of June 2024, leaving almost nothing free. An 0800 number is 11 digits (0800 plus seven), so the pool is far larger and it is easier to obtain a number close to the one you want.
0120 numbers are obtained by companies, but they are also used for sales calls. If an unfamiliar 0120 number calls you, it is safer to check who it belongs to before calling back. See also the difference between 0120 and 0800, Free Dial versus Navi Dial and the business benefits of Free Dial.