Applicability of sampling (clause 7.3) to a calibration laboratory

You asked 

in what circumstances Sampling (Clause 7.3) is applicable to a calibration lab? In the normal course, a calibration laboratory is supposed to calibrate (within its scope) whatever MI is sent to it by a customer. Is my understanding correct?

Yes, your understanding is correct. For a laboratory re receiving items for calibration, there is no sampling involved. The lab receives the item/s referred to typically as “unit under calibration”, “unit under test” or “device under test”

You also asked

Another view is that calibration lab can apply sampling for its internal quality control purposes. For example, it can randomly sample MI calibrated itself and recalibrate them using another equipment/method/calibration technician etc. Is 7.3 applicable in this case?

No, clause 7.3 is not applicable in the case described. ISO 17025 clearly refers to sampling as an activity that leads to subsequent testing or calibration. This means, for the purpose of releasing results.

For more information on the requirements of ISO 17025, download the free White Paper Clause-by-clause explanation of ISO 17025:2017 at https://info.advisera.com/17025academy/free-download/clause-by-clause-explanation-of-iso-17025

You can also have a look at the ISO 17025 document template previews: Sampling Procedure as well as the two appendices, at  https://advisera.com/17025academy/documentation/sampling-procedure/