SQL Help - Exclude data with just one distinct value
One way to do this:

with stats as (
  select agentname,CallCenterID, count(leadid) Dials
  from LeadCallHistory
  where agentname not in ('vdad','system')  
    and cast(callCenterStartTime as date) between cast(getdate()-1 as date) 
    and cast(getdate()-1 as date)
  group by agentname, CallCenterID
) 
select * from stats s1 where exists (
  select 1 from stats s2 
  where s2.CallCenterID = s1.CallCenterID
    and s2.agentname &lt;&gt; s1.agentname
)