You only need to register as a telemarketer in the states where you make calls, provided that those states require registration. 32 states and the District of Columbia currently require registration. 

Those states are as follows: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Delaware, Florida, Idaho, Indiana, Kentucky, Louisiana, Maine, Mississippi, Montana, Nevada, New Jersey, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Dakota, Texas, Utah, Vermont, Washington, West Virginia, Wisconsin, and Wyoming.