Re: shared office phone and the company wants to track the employee it is currently loaned to?
If the phone is owned by the company, it is free to install its own tracking software to locate it at any time.
Ah, but that's a very different scenario. If the employee knows the tracking app is there, then they are implicitly consenting to be tracked if they borrow the phone, and if they don't know it is there you're entering fairly murky waters of employment law.
Neither situation is really covered by GDPR, which explicitly protects the PII of a person, not of an object which they may or may not have with them.
it is still no reason to prevent the owner being granted all location data held by the telco
If the telco definitely has location data on the owner, that would be true, but if the owner can't prove that the data identifies their location I would argue that GDPR doesn't require it to be released. It may be someone elses PII, and that's usually enough for the courts to arguable that there is reasonable doubt.