Is it permissible to randomly write a landline number or the number of a relative when registering at universities or online, if the landline field is mandatory and the registrant does not own a landline?

If registering a landline number is mandatory, the number does not have to be your own. You may provide a number belonging to a relative or a friend, after seeking their permission, and this is not considered deception. However, providing a random number is not permissible, as it constitutes falsification and lying, and may cause annoyance to the number's owner.