Unknown Number – Season 1

The story follows He Jun, a hard-working detective who meets a young and passionate criminal profiler named Gu Ming Zhe. Gu Ming Zhe's life begins to change when he unexpectedly finds a mysterious old phone that connects him to a certain Cao Zheng, a clumsy yet resolute detective who unbeknownst to Gu Ming Zhe, has been missing for 20 years and is calling from the year 1998. The team then takes on the task of resolving impossible cold cases. (Source: DramaPanda)