Hi,
I suggest that you search the internet for any number of guides on using ldapsearch and writing queries like this.
Given that I don't know which attribute you store the telephone number in for your site, it is hard to provide a definite answer... in general it would be something like:
ldapsearch -h <directory server> -b <organisation base> -D "cn=directory manager" -w <directory manager password> '(|(uid=*)(telephone=*))' uid telephone
Regards,
Shane.