Samsung A640 - Manual (Page 238)

216 Section 3B: Sprint PCS Vision
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
Sprint PCS Vision 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 > PCS Vision > Disable Vision in your phone’s
menu.