Validation to be done in code behind

I have two textboxes(one for mobile number and one for phone number).Following validations neeeds to be done in code behind. [1]one of the two(from mobile number and phonenumber) needs to be there. [2]If phone number is enetered then std code needs to be enetered also.(There is textbox for std code) I would highly appreciate if somebody could provide elegant code for this. Thanks in advance for replying(as always) ...