Beginning August 18, 2018, a new dialing procedure will take effect that requires callers to dial the area code + telephone number for calls within the 609 area code region.
This means that all calls within the 609 area code, that are currently dialed with seven digits, will need to be dialed using 10 digits: the area code + telephone number. The same dialing procedure will apply to telephone numbers assigned to the new 640 area code from which new telephone lines or services in the 609 area may be given beginning September 17, 2018.
In addition, calls made from the 609 area code to the 856 area code (southwestern New Jersey), that are currently dialed with seven digits, will need to be dialed using 1 + area code + telephone number.
More information can be found here. Please be sure to include the 609 area code when trying to reach Mercer County.