You can book online through either of the airlines websites or you can come in person and book at the ticket counter. The ticket counter is open an hour after each scheduled flight departure for people to come in and book tickets in person.
I'm not sure what you mean by call but I'm going to assume you are referring to the scheduled commercial service in which case you would book your ticket with the airline just like flying out of any other airport. If you're worried about the flight selling out the answer is always going to be sooner rather than later. As demand for flights fluctuates throughout the year based on regional weather and events both locally and at the destinations the airlines serve there's no fixed answer for how long before your travel date to book to ensure there's space for you. If you need more specific information I would advise contacting either Allegiant or Frontier (the two airlines that serve Portsmouth) and talk to one of their agents. These companies do not typically monitor the questions sections of the airports they service.
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