Have you tried checking the phone number on the offer ties up with their offices? If so, then tyou could always 'phone the HR department with a harmless question such as "Can you confirm the start date?" or "Do I need to arrange anything before I start, such as a medical?". They won't mind being asked this sort of question as it makes you look positive.
If you applied via an agent, then contact your agent & ask them for confirmation.
Have you tried putting the phone number into an internet search to see if anything 'odd' shows up.
Just employ some lateral thinking, believe me it will be expected if you join them ;-)
Good luck (hopefully in the job)
Last: P&O Oceana December 2016
Next: NCL Jade August 2017
Then: P&O Ventura December 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(4), Oriana(12), Ventura(2), Victoria(5)
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Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey, CMV Magellan