1.check for look and feel of the phone.
2.check for numbering order.
3.check wheather phone is connected to internet or not.
4.if it is connected to net,dose it get dail tone or not.
5.while dailing numbers do you get beep sound or not.
6.after dailing number can u hear ring or not.
7.when your end cal ,it is disconnecting or not.
8.if you press redail,it is again connecting previous 
number or not.
9.check for length and width of the phone.
10.check for the material used for phone.
11.check for the design of the phone.

Positive Testcases for an Ip phone :
1. When the Ip phone is connected , is the dail tone 
correct on pressing a valid number.
2. Does entry of a valid number make a  call placement.
3. If there is an invalid number entered is there 
appropriate message duisplayed.
4. Make an incomming call to the Ip phone does the Ip phone 
display the caller correctly?
5. Does the incomming call produce proper ring tones 
6.Is there a voice jitter when a call is made from the Ip 
phone/
7. Is there a voice jitter when a call is from external 
number
8. does the call get registered in history.
9. Am i able to store numbers 
10. Are the various ring tones working fine
11. Is a conferenced call sucessful?
12. Is the call transfer sucessful


The above are just a few of the Positive testcases there 
are many more cause VOIP is a vast field. 
Additionally there are several negative testacses too that 
can be tried.