It depends on what state you live in. Alaska, Colorado, Florida, Illinois, Maine, Massachusetts, Michigan, Mississippi, Missouri, New Hampshire, New York, North Carolina, Ohio, Tennessee, and Vermont allow phone mounts anywhere on the windshield.
Arizona, California, Hawaii, Indiana, Maryland, Minnesota, Nevada, and Utah allow phone mounts in specific locations on the windshield. Usually this is near the bottom, but exact location varies by state.
The following states don’t allow phones or GPS units on the windshield. Alabama, Arkansas, Connecticut, Delaware, District of Columbia, Georgia, Idaho, Iowa, Kansas, Kentucky, Louisiana, Montana, Nebraska, New Jersey, New Mexico, North Dakota, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Texas, Virginia, Washington, West Virginia, Wisconsin, and Wyoming.
That said, regulations are always changing, so be sure to check on your state’s laws regarding phone mounting.