Hi Keith Porthill,
The guidance at NMWM07160 advises that where there are two or more employers, each should be considered separately, to ensure the NMW applies whilst in their employment.
NMWM07160 - Types of work: working for two or more employers
The terms “on call” and “stand by” are commonly used to describe an arrangement where a worker makes themselves available to respond to work as and when required.
NMWM08170 provides general advice on NMW and being 'on call' or 'stand by'.
NMWM08170 - Working time: salaried hours work: on call and stand by
For advice in this particular area, please have a look at:
Employers: National Minimum Wage enquiries and complaints