Telephone Services
Telephone time-provider services require the time-provider to dial a centralized UTC time source through a modem. Modem speeds and line delays can affect the accuracy of the time returned.
Telephone services are usually provided by standards agencies. For example, in the United States this service is offered by the National Institute of Standards and Technology (NIST). There is a per-call fee for the service in addition to the cost of the modem software.