Django models - field dependency
Case 1:
Don't do it like that, have a different table for telephone numbers and have a ForeignKey from the Person (I'm assuming it's a person) to the the telephone number. If you have more than one telephone number per person, do it the other way around, otherwise consider using a OneToOne.
Obviously you'll want the ForeignKey to be nullable. That way, the only way to have a telephone number is if the person provided one.
Case 2:
I don't understand your database design here, so I can't answer. You'll have to explain more - why do you need a ManyToMany here?
How can I have field dependency?
Case 1: If boolean field call_me is set, then telephone must be set, else it should be blank
Case 2: If many to many field category (with values sale, rent) has one of the values as sale, then price_sale must be set, else it should be blank