There are several reverse phone-lookup services available online. However, not all phone numbers can be successfully searched using these services. Landlines and business numbers are most likely to produce results, while unlisted or newer numbers and cell phone lines are less likely.
The reason cell phone numbers are more difficult to search using reverse lookup is that unlike landline numbers they are not issued by an interlocking system of regional phone companies. Rather, cell numbers are issued by individual providers, each of which has its own policies about releasing customers' numbers. There are a few reverse lookup services specifically for cell phones, but these databases are generally not free to access.