Ithicos Solutions applications do not currently support multi-value fields though some fields in the Active Directory are multi-value fields. This includes the "other" phone number fields. You can see a good example of this in Active Directory Users and Computers on the Telephones property page.
The phone number fields on this page are all single valued fields, but next to each phone number is an "Other" button. If you click the Other button, this opens up an editor for a multi-valued field.
For example, if you click the Other button next to the "IP Phone" field, this will open the editor for the otherIpPhone attribute.
Directory Update and Directory Manager treat these fields like single value fields. If you enable either of these applications to edit one of these fields, they will overwrite the existing data and replace that data with only a single line of data.