question

We have a ringcentral pro account.  Under "company numbers and info" It has one main number which is voice and fax (We only use fax) and one other number that is a fax number.   I would like to add another fax number.  When I go into add another number, it will let me add another number but doesn't give me the option to say its just a fax number.  I went right through to the confirm screen where you confirm the cost to add another number and no where could I say that number is a fax number.  What am I missing?