A ground telephone number where you can find a copy of the NOTOC should be on the NOTOC form since dangerous goods information must all be contained in one single document which is the NOTOC. The telephone number should not be on other forms or at other places.
One single document is sufficient to look for in an emergency. The entry of the telephone number is not mandatory. It is up to the individual airline. Where there are many dangerous goods on board such as with a freighter aircraft, it is more efficient, convenient and time saving if the phone number is shown on the NOTOC.