Can anyone explain why the following is happening?

We are using Option GPRS in a handheld device which is running Windows
CE.Net 4.2, if we try and use a card with a serial number prefix of
"WK" then the connection will not dial unless we specify the "Use
terminal window before dialling option". However if we use a card
prefixed with "WP" the connection is made with no problems. Has anyone
else encountered this problem or preferably could anyone explain why
this happens?

Thanks. John.