A national calling frequency in the USA is a radio frequency that is widely accepted and understood to be a place to start communicating with other hams.
This frequency is established for each RF band under the voluntary US band plan and is not regulated by the FCC. It is routinely monitored by any number of radio amateurs and is likely to result in a response when calling CQ or Mayday or SOS.
The US band plan shows calling frequencies for various modes (CW, SSB, FM, AM, digital) in different bands. A calling frequency list includes not only the modes but adds specific activities (expeditions, power levels) as commonly practiced.
For new hams who are likely to get started in local VHF/UHF operation, the national calling frequencies to be concerned with are 2m and 70cm FM simplex (non-repeater). These are 146.520MHz and 446.000MHz, respectively, and should be included in your radio’s scanned channels. If you regularly use 1.25m, 33cm and/or 23cm bands, there are national simplex calling frequencies defined per the band plan for you to look up.
Be aware that the national calling frequency for whatever mode and activity is only a place to start communicating. Protocol and common courtesy require that once contact is established, you move to another frequency (QSY) to leave the calling frequency open for others to use.
The national simplex calling frequency is sort of a universal (within the USA) place to make contact when you are not sure where to start. It is particularly useful when traveling to a new area where you don’t know the repeater frequencies, offsets and access tones. It’s also a great place to get assistance in an urgent situation.
Note that Canada largely follows the US national calling frequencies so our friends north of the border can be included in this scheme. Other countries may or may not have national call frequencies, or may unofficially follow the US plan.
This may also be a good time to remind all hams that while even the Technician license grants full privileges on VHF/UHF bands, and your 2m FM transceiver tunes from 144-148MHz, doesn’t mean you can just randomly pick a frequency and key the mic.
For one, chances are nobody will respond because it’s unlikely anybody is monitoring your random frequency. But mainly it’s because there is a clearly defined band plan for 2m and it’s not all FM simplex and repeater allocations. Take a look at it:
You can see that only a portion of the band is allocated for FM simplex or duplex (repeater) work. There are clearly-defined chunks for CW, SSB, beacons and satellite operation. You need to stay away from those frequencies unless you’re operating those modes and activities per the plan.