IANAL but this does seem pretty straightforward in broad terms.
the problem is not critical we can proceed with the development without fixing it.
First note that this is your opinion but it is your boss who has the right to make the decision. Your boss can over-rule you and I'd suggest getting used to that. Your opinion, in this case, would almost certainly not be a good enough reason to refuse a direct order to contact "Bob". Also remember that your boss may regard it as critical for reasons you are not privvy to, and may be thinking of a larger business context than you are.
Can you be forced to do it ? No.
Can you be fired for refusing an order to do it ? Possibly.
Can you be disciplined without being fired if you refuse the order ? Almost certainly.
Is it legal to ring "Bob" on vacation. Yes. Unless "Bob" has a court order preventing someone specific or a specific organization from ringing him, he's fair game for anything short of abusive and/or harassing calls.
Could it violate "Bob"'s contract of employment ? Maybe, but that's a matter of a contract dispute between "Bob"'s employer and him. It's not an issue for you in any way if you are acting under a reasonable order (which it would be).
So ring "Bob", say "Sorry, I was ordered to ask you this." and get on with it.