The 手机号 is arguably one of the most important pieces of information a person has in modern China, far more integral to daily life than a cell number in most Western countries.
Digital ID vs. Contact Info: In the West, a phone number is primarily for calls and texts. In China, a 手机号 is a unique identifier required for “real-name registration” (实名制 - shímíngzhì). You must link your government-issued ID (or passport for foreigners) to your number. This number is then used to register for virtually every app and online service, from social media (WeChat, Weibo) to e-commerce (Taobao) and banking (Alipay). It functions as a de facto national digital ID.
Social Connection: Asking for someone's 手机号 is the modern equivalent of asking for their contact information. However, the primary goal is often not to call them, but to add them as a friend on WeChat (微信 - Wēixìn), which is China's ubiquitous “super-app.” Many users link their WeChat account to their phone number, making it the easiest way to find and connect with them.
Numerology and Status: Numbers have significant meaning in Chinese culture. A 手机号 with lucky numbers is highly desirable. The number 8 (八 - bā) sounds like “to get rich” (发 - fā), while 6 (六 - liù) sounds like “smooth” or “fluid” (流 - liú). Conversely, the number 4 (四 - sì) sounds like “death” (死 - sǐ) and is avoided. People will pay significant premiums for a “lucky” phone number, as it is seen as a status symbol.