Hm. It is hard questions because it is not so easy to choose appropriate ITSM system. It depends on level of your company and requirements, that ITSM system should match in order to satisfy your needs. The similar software from Tivoli and Remedy are pretty expensive.
Probably, you may try OTRS. OTRS is an Open source Ticket Request System (also well known as trouble ticket system) with many features to manage customer telephone calls and e-mails.
http://otrs.org/
Also you may try Jira:
http://www.jira.com/index.html
Jira is not free, but fairly interesting.
"HelpDesk Advanced" also looks pretty powerful:
http://www.pat.eu/prodotti/helpdesk-eng.php
And the last: if you do not know what to choose - you may try Bugzilla:
http://www.bugzilla.org/
Bugzilla is free open source system, that might be used either as bug tracking system or as ticketing system.
Hope, my suggestions above will help you to make a right choice.
Regards,
Max