Didn't know that, was it ever mandatory? It was the 1970's when I was there.
Only the service in the IDF is a must, when the Israelis are 18 years old (except some Haredim with religion problems).
In the age of 17 every Israeli gets the first order to the IDF's recruiting office. There they are passing medical checks to put a military profile, interviews, psychotechnical tests and other things.
After that, according to the tests results and medical checks and interviews, they get options to choose a duty that they would prefer to serve at. Some get their wishes, while others dont (depends of course on their physichal and technical abilities).
Those who can serve at combat and elite forces try to pass many other tests so the IDF will decide where to mobilize them.
Others dont know where they are going to serve until they get the mobilization order in the age of 18.
Only when they start their service on the day the mobilization order says, their first trainings start.
Hope I cleared something.