I assume that you are using the directory as provisioned by 3CX. In this case the protocol behind it is not LDAP. By default, 3CX is provisioning the address book of the Snom phone. You should be able to see all contacts if you log in to the phone and click Directory on the left menu.
The problem with the sorting is that 3CX is provisioning the entire name (First name + Last name) as only one field in the Snom Address Book (the field is called Nickname), so basically the only sorting possible is by Nickname. You can see this in the phone's web interface by clicking thew Edit icon on one of the entries in the Snom contacts list.
The only idea I have to solve the sorting is to contact 3CX Support and ask if they can change the template and provision the address book fields First Name and Family Name separately instead of both together under Nickname. Or you could as well set up and manage your own LDAP server or address book independent from the one provisioned by 3CX.
I hope this helps.