What can I do if my job told me I would get paid for a mandatory conference call but they aren’t paying me?

If you were an exempt (from overtime) salaried employee, then they do not need to pay you: salaried exempt employees can be made to work any number of hours (including on conference calls) without any additional compensation beyond their weekly salary. Even if they said they would pay you, they don't have to, since there was no "consideration" to turn their promise into a contract: that is, you had to be on the call anyway (your employer can order to be on a call), so you gave them nothing in exchange for their promise.

If you were salaried but not exempt from overtime (to see the test for who is exempt, go to the U.S. Department of Labor, or DOL, website; there are 4 or 5 categories of exemption, and you must fall  under at least one in addition to being paid on a salary, not hourly wage, basis), then IF the time spent on the call meant you worked more than 40 hours that week, they should pay you overtime (or an amount equal to 50% of your effective hourly  base rate) for overtime hours. But if you worked 40 or less hours, they owe you nothing beyond your salary.

If you were paid on an hourly basis, you must be paid for the time spent on the conference call, which is work; and be paid overtime if you worked more than 40 hours that week. That is the law, so it doesn't even matter whether they promised to or not.

However, if your employer owes you money but will not pay you voluntarily, the only way to get compensation would be to sue them. Since suing your employer has obvious costs, both monetary and otherwise, it may not be worthwhile to do this.