7.1.5.3.2 states that if the calibration is not performed by a 17025 accredited lab that "there shall be evidence that the
external laboratory is acceptable to the customer."
The Note in 7.1.5.3.2 says "Such evidence may be demonstrated by customer assessment, for example, or by customer- approved second-party assessment that the laboratory meets the intent of ISO/IEC 17025 or national equivalent."
This means the customer either needs to have a second party assessment of your calibration system, or they are required to do that themselves. Most customers are not going to be able to demonstrate that your company's calibrations meet 17025 requirements, as they do not have the expertise or time to perform what amounts to be a 17025 audit of your calibration program. They must produce the evidence that this assessment has been performed, otherwise they will be found to be non-compliant.
As a result they will go to 17025 accredited suppliers.