1. Check for the Color.
2. Check for the Available buttons.
3. Check for the size of the buttons
4. Check for the distence b/n the buttons.
5. Check for the Size of the telephone
6. Check for the Weight of the telephone.
7. Check for the meterial which is used for manufacturing 
the telephone.

Etc..

First of all we need to test the functionality then comes 
the UI so we need to check whether the phone is ringing or 
not then we can move on to the next things.Telephone cable 
properly inserted in the hand set and the socket like that

1. check first phone is visible or not
 2. check for that phone colour.
 3. check for size of that phone.
 4. check for phone is in working condition or not.
 5. check that phone weight. etc..,,,

1.here we check functionality only, no need to go colors,  
weight etc...
2.check the buttons like press redial, *, flash buttons...

Valid Test Cases:

1.Lift the handset, u should get a tone.
2.Dial the number again u should here the dial tones
3.If the number is busy, in active, or any such issues you 
should get an alert about the same
4.If no such alerts , you should here riging tone in the 
hand set.
5.If it rings for a long time n destination doesnt picks up 
the call, an alert should be given.
6.If not the above case, at destination user must lift the 
phone.
7.When he lifts the phone ringing tone must be ended.
8.Now talking session must take place.
9.Both src and destination user can talk n hear properly.
10.If any one of them closes the session, again a tone must 
be heard at the other end.
11.Close of session between the 2.

Now u can try for Invalid test cases. ;)

Lift the Phone

Check for beap sound if yes, start dialing
Check whether you will be able to hear the ring 
Then Check whther u will be able to hear the voice of the 
user....

Here u have to Split the Functinality of phones and start 
writing test cases.. Don write test cases for Entire 
Telephone. its stupidity.....

I have told test cases for the functionality of the phone
(To check if its working or not) if any spelling mistakes 
forgive me.. I have typed it fastly

1. Check whether the phone is connected via a telephone 
line.
2. Check for dail tone.
3. Check the speaker volume by increasing/decreasing.
4. Check the keypad.
5. Check the display of numbers in the monitor while 
pressing keys.
6. Check redail button.
7. Check forward call button.
8. Check the volume of the other user.

CALL FUNCTIONS
Depends on the functionality/rules applied to the phone - 
what the users are supposed to be able use the phone for.  
The above are all valid but what I think is missing is 
tests like the following - actual call tests and expected 
results: 
Dial an 0800 number - e.g. should work
Dial a local number
Dial a national number 
Dial various cell phone numbers
Dial an international number - e.g. should not work, is 
toll barred etc
Dial an 0900 number
Dial an internal extension
I would steer away from testing every aspect of the phones 
functionality such as call forward and redial but just a 
simple test of the main ones such as:
Use the phone directory (if an internal directory exists)
Use speaker phone
Use mute
Try with a headset