The reason is capacity, not caution. A lead called back in ten minutes is a different conversation from the same lead called that evening. Buy more than you can call quickly and the extra leads go cold in your CRM, which is the most expensive way to find out you needed another person on the phone.
There is no minimum and no lock in, so the first batch is a test rather than a commitment. Agree the criteria, agree the fixed price per lead, connect your CRM and start. The full sequence is on the page about how to buy mortgage leads.