The commission of the watchman is to sound the ‘Teruah’ - the warning call of the Shofar. This is what the scripture says:

 

..if the watchman sees the sword coming and does not blow the shofar, so that the people are not warned; and then the sword comes and takes any one of them, that one is indeed taken away in his guilt, but I will hold the watchman responsible for his death.

 

Ezekiel 33:6