Group By Error

Can anyone tell me why I'm getting this msg, or even tell me what it means;
Thanx,
Trudye

SELECT C.[TimeStamp] As [TimeStamp], CustName AS CustName,
LTRIM(RTrim([Phone])) AS Phone, FirstOfExplain_followup AS FirstOfExplain_followup,
RetailerName As RetailerName, Issue As Issue, FaxContact AS FaxContact,
ResultsKey As ResultsKey, Real_Client As Real_Client, D.callback As Callback,
@CboClient As CboClient, ResultsDepartment As ResultsDept, [Status] As [Status]
INTO tmpE_C_Report
FROM tmpE_C C
INNER JOIN tblEData D ON D.ResultsPKey = C.ResultsKey
GROUP BY C.Timestamp, [CustName],
Phone, FirstOfExplain_followup, RetailerName, Issue, FaxContact, C.ResultsKey,
Real_Client, callback, @CboClient, D.ResultsDepartment, [Status]

Msg 164, Level 15, State 1, Procedure Escalation_Callbacks_Rpt_5, Line 155
Each GROUP BY expression must contain at least one column that is not an outer reference.

Thanx,
Trudye