First Assume what is the purpose of mobile handset-then you 
will get an idea of what you want to do-then convert those 
ideas as test scenarios-then start writing test case for 
that scenario.

Example:you use mobile for calling somebody--for that what 
you do-you either search from existing contact list or you 
just enter number-now what is the flow of action ,write as 
steps.

Step1:User enters the number by pressing the numeric keys 
of handset.
Expected result:number gets displayed simultaneously as the 
user presses the button.

Step2:User then clicks the call green button.
Expected:Connecting symbol should appear on the screen with 
simultaneous audio signal and shows connected symbol once 
connected.

and in this way the flow of steps can be continued 
depending on the scenarios defined.

For example take another scenarios of adding contact to 
mobile

Step1:User presses the menu option or shortkey defined.
Expected:User is taken to menu options

Step2:User selects the address book.
Expected:Address book menu is displayed with sub menu 
options as add contact,delete contact,view contact etc.

Step3:User selects the Add contact.
Expected result:Add contact screen is opened requesting 
user to enter various fields like Name,number,add picture 
etc.

Step3:User enter the mandatory fields and clicks on the 
ok /chooses the save option button
expected result:New contact is added successfully and 
displayed in the contacts list when searched by the various 
options.

and so....on the teststeps are continued.