Recognise that outsourcing some tasks might be more efficient. Especially if your organisation is struggling to recruit enough people to fill needs (in a tight market).
Paying an external expert to do some specific tasks (e.g. calibration of sensors) may be quicker (saving costs and time) than finding an internal commissioning engineer with available time and getting them trained.
Also recognise that such training could benefit the company in the long run if it is a common task. If not, then outsourcing it may be the best way to get it done.
Time can often be the biggest saving to the project by outsourcing some tasks, as long as you manage the risks of doing so properly.