That's a very weird view of what a salaried position means. I'd even say very US-centric, since most other developed countries consider a salaried position as being paid for a certain amount of hours worked per month, there are limits on hours worked because if not there's nothing limiting the employer on requesting you to work as many hours as they please ("I pay you a salary, of course you need to work 14h/day").
In most parts of the globe, yes, you are working for free in this case, your contract does not stipulate you are available to work at all times, or more than the allocated hours per week/month of your contract. If there's a need for work outside of these hours they are considered overtime.
On top of that, on-call work disrupts your life outside of work, you can't do anything that requires complete disassociation from work life since you can be paged at any time, not paying a premium for this type of alertness and labour is pure exploitation.
