Recently, many mobile applications where developed to enable VOIP call between cell phones,
Currently Only Symbian OS mobile phones and iPhone support voice calls, other java enabled phones still can IM, send voice messages and pictures for free!
One of the many applications in this domain is Nimbuzz. Nimbuzz has 2 versions one for java enabled phones and one for symbian OS phones.
Both versions support IM between Nimbuzz phones, IM with Yahoo, MSN, Gtalk and others, Sending Voice messages and directly from camera pictures. Also nimbuzz can Buzz a friend that is not currently running the software telling him that someone is waiting for him on nimbuzz, Also you can start international calls with the price of local calls if Nimbuzz had a server in your country.
The symbina version of nimbuzz adds the possibility to start and internet VOIP calls for free (only internet download fees apply)
Also nimbuzz can be useful to backup your phone contacts on the application server, and then restore them on another phone!.
You can check out the Nimbuzz website here