How SQL query are evaluated
Why don't you use an OR condition? 

select * 
from sip_trunks 
where resporg_account_id_id = 62 
  and (   sip_trunks.default = true 
       or identifier = (select action 
                        from sip_trunk_numbers 
                        where number = 'xxxxxxxxxx' 
                        and resporg_id = 80 
                        limit 1))

limit 1




If you need to know which condition matched, you can use a UNION:

select *
from (
  select st.*, 1 as source
  from sip_trunks st
  where resporg_account_id_id = 62 
    and identifier = (select action 
                      from sip_trunk_numbers 
                      where number = 'xxxxxxxxxx' 
                        and resporg_id = 80 
                      limit 1) 
  limit 1

  union all

  select st.*, 2 as source
  from sip_trunks st
  where resporg_account_id_id = 62 
    and sip_trunks.default = true 
  limit 1  
) 
order by source 
limit 1


If source equals one in the result the first query matched, otherwise the second.



Alternatively you could use an outer join:

select st.*, 
       stn.action is not null as identifier_matched
from sip_trunks st
  left join (
    select action 
    from sip_trunk_numbers 
    where number = 'xxxxxxxxxx' 
      and resporg_id = 80 
    limit 1  
  ) stn on stn.action = st.identifier
where resporg_account_id_id = 62 
  and (stn.action is not null or sip_trunks.default = true)
limit 1


Note that limit without an order by usually makes no sense.