i got what you r asking for..the call summary is not currently implemented and i cudnt find an app for it..do PM me if you happen to find one..but u do have an extended call log to track the calls u have made incase u r interested but it wont solve your purpose of checking tha balance...

the automatic notification that comes from the operator doesnot work on n900 bcos ussd dialling is not yet working..the balance msg you get when u end a call is a response to a automatic ussd which is send to the operator when you end a call..since n900 currently does not catch the ussd responses so forget that till the next release atleast..hope u getting what am trying to say

in the meanwhile to check your balance,free sms n free mins u can use the ussd widget..its pretty easy to use n has been very helpful for me..