ISO 17025 accredited labs and services

You asked

Are there any sorts of items that don't necessarily need to be sent out on a regular basis?"

Yes, certain equipment need not be calibrated by an external ISO 17025 calibration laboratory. Furthermore, the frequency of external calibration can vary.

Based on your knowledge of your method, if a particular piece of equipment could influence the results, it must be calibrated. You need a certificate with reported uncertainty. This is so that you can be confident that the method performance is still valid (as per validation studies) and that the measurement uncertainty of the device and contribution to the uncertainity of the test result is known.  If there was no confidence in the consistent performance of equipment, there is no confidence in the validity of the test results. To ensure valid results and metrological traceability of measurements, there needs to be an unbroken chain of calibrations and known controlled contribution of uncertainty of a device to the overall Measurement uncertainty of the test method.

For more information and important reference links, have a look at a similar Q&A at https://community.advisera.com/topic/re-calibration-time/