I think you can a custom filed from the back end if you are admin.
1. login in to backed using admin.
2. Goto listing -> custim field.
3. press add new.
4. Click the edit on right side of the "NEW custom field".
5. Give a name - some thing like "contact no"
TYPE - TEXT
6. select catagories and save.
This should work.
But this field is a text field so dont expect much. !
Now the interesting part is that. Once you open "publish new item" from the front page the contact number field will not appear at first.
But it will appear once you select the corresponding catagory.
you can check my example. here
http://buzdubai.com/classifieds/item/new