ROQL count contact records with unique phone numbers

Question

I'm a bit new to using Oracle service cloud. Here I'm trying to query the number of contact records with a unique phone number also email address is not null using Customer Portal's Connect Object Explorer. Also I find ROQL bit confusing in some cases.

Is there a better way to get this number in my Oracle service cloud site?

Contact table :

Table Label       : Contacts 
Table Description : Contains information on the contacts in your knowledge base. 
Tbl               : 2 
Visibility        : Service, Sales, Marketing, Surveys, Chat 
Custom Fields     : Allowed 
Attributes        : Pseudo Table 
Primary Key       : c_id 
Columns : AND/OR (search_and_or)
          Contact ID (c_id)
          Email Address (email)
          Default Phone (ph_asst)
Indexes : c_id (Unique) 
          email
          ph_asst

Also I'm interested to find out whether it can be achieved by this endpoint by passing the query.

Any suggestion would be highly appreciated.