Updated: Jan 27, 2021
Metal roofing as a standalone product does not have noticeable adverse effects on cellular signal. If there is an existing nearby disruption to cellular signal (i.e., electrical station) then a metal roof may amplify those effects, however, any thick material will disrupt cellular and other RF signals. This includes, wood, concrete, clay tile, asphalt and many other materials. This effect is why you momentarily lose cellular signal when driving through a tunnel. In fact, metal roofing is sometimes blamed for poor cell reception when other materials may be playing a bigger part in repelling RF signals. For example, fiberglass insulation, while not very dense, does an excellent job of repelling RF signals. The same materials it contains to keep heat and cold in, act to repel RF signals. While it may seem like an instinct to move toward a window when you have poor signal, most windows are even coated with energy efficient coatings like metal oxide that will repel RF signals.
The truth is, you have likely used your cell phone numerous times in structures with metal roofs and probably haven't noticed any change in signal strength. Many churches, banks, malls, stores, and other buildings have metal roofs or flat roofs with steel decking. The majority of complaints regarding loss of cellular signal with a metal roof come from areas where cellular signal is already unreliable. If you currently have satisfactory cellular signal then installing a metal roof won't cause any noticeable changes to the reliability of your cellular service, and shouldn't factor in to your decision to purchase the roofing material that you want.
If you live in a home with a metal roof or other thick roofing material that is impeding your cellular signal, you may consider purchasing a cellular signal booster to help gain a stronger signal. With the majority of American homes being connected to a broadband connection you are also likely able to make calls with WiFi calling if your device is connected to your home network.