the watchman opens = The type of fold spoken of here is a communal fold close to town in which several flocks would be kept for the night under the watchful eye of a doorman. The walls were high and the wooden door would be bolted from the inside and only opened to the right shepherds. Only those with a right to enter were allowed to do so.

his own = The shepherd does not call sheep in general; he only calls those which belong to him. He knows each of them and has a name for each. In Palestine sheep are usually kept for their wool and thus are in the flock for several years. The shepherd gets to know them quite well and they become familiar with him and his voice. Often the sheep have names which are based on physical characteristics (i.e. “brown-ear”). At any rate, they know his voice and, unless distracted, obey his call. Note the following account from the mid 1800’s:

“As we sat the silent hillsides around us were in  a moment filled with life and sound.  The shepherds led their flocks forth  from the gates of the city.  They were in full view, and we watched them and  listened to them with no little interest.  Thousands of sheep and goats were  there, grouped in dense, confused masses.  The shepherds stood together  until all came out.  Then they separated, each shepherd taking a different path, and uttering as he advanced a shrill peculiar call.  The sheep heard  them.  At first the masses swayed and moved, as if shaken by some internal  convulsion; then points struck out in the direction taken by the shepherds;  these became longer and longer until the confused masses were resolved into  long, living streams, flowing after their leaders. … The shepherds themselves had none of that peaceful and placid aspect which  is generally associated with pastoral life and habits.  They looked more  like warriors marching to the battle-field —- a long gun slung from the  shoulder, a dagger and heavy pistols in the belt, a light battle-axe or  ironheaded club in the hand.  Such were the equipments; and their fierce  flashing eyes and cowling countenances showed but too plainly that they  were prepared to use their weapons at any moment.”  J. L. Porter, A.M., in “The Giant Cities of Bashan,” 1867.

H. V. Morton tells of a similar account:

Early one morning I saw an extraordinary sight not far from Bethlehem. Two shepherds had evidently spent the night with their flocks in a cave. The sheep were all mixed together and the time had come for the shepherds to go in different directions. One of the shepherds stood some distance from the sheep and began to call. First one, then another, then four or five animals ran toward him; and so on until he had counted his whole flock.

In the Steps of the Master, London, 1935, p. 155