Samsung SPH-M350 Online Manual page 159

When is my data connection dormant?
If no data is received for 10 seconds, the connection
goes dormant. When the connection is dormant, voice
calls are allowed. (The connection may become active
again quickly.) If no data is received for an extended
period of time, the connection will terminate.
Can I sign out of data services?
You can sign out without turning off your phone;
however, you will not be able to browse the Web or use
other data services. While signed out, you can still
place or receive phone calls, check voicemail, and use
other voice services. You may sign in again at any time.
To sign out, go to > Settings/Tools > Settings >
in your phone's menu.
More... > Data > On/Off
Section 3B. Web and Data Services 153