Original sentence: Although the NMI became a commonwealth in 1978 and the United States assumed sovereignty in 1986, calling between the NMI  and the rest of the United States remained an international call (+670-XXX-XXXX)
Edited sentence: Although the NMI became a commonwealth in 1978 and the United States assumed sovereignty in 1986, calling between the NMI  and the rest of the United States remained an international call (011-670-XXX-XXXX)