acc:acc_aaa_request: failed to add Contact, 17

Hello: When I am do accounting with opensips1.6.4+freeradius2.1.10+radiusclient0.5.6,I meet the following error: acc:acc_aaa_request: failed to add Contact, 17 I kmow I need to define the attribute "Contact"in the dictionary file "dictionary.opensips",but I can not find the value and type of the attribute "contact",can anyone tell me the value and types of the attribute?Thanks a lot.