The answer is different according to whether the advertisement you are looking at is Classified or Verified.
For classified or non verified landlords
By default the contact details of the owner of a classified (or "non-verified") accommodation are hidden.
If you wish to be able to see all the contact details of all non-verified landlords on Erasmusu you can purchage a premium subscription and their contact details will be made available.
If the landlord has already purchased a premium subscription on their side, their contact details will be made public so will be visible to you. However contact details of all other non-verified landlords will still be hidden.
If you are trying to contact a verified landlord, you can send messages through the Erasmusu messaging system. No external contact details are shown until a booking is made on Erasmusu.