I work as an Assistant Property Manager, I am non-exempt and I work a regular 8am-5pm, Monday-Friday schedule. However I am also on call from Thursday-Wednesday every other week. Being on call for my job means that I take the calls from our building tenants that come in anytime before 8am or after 5pm. Often I get called in the middle of the night or on weekends. Some of these calls only take 5 minutes over the phone, while others require me to go to our properties.

My employer is telling me that if I get a call, it must take more than 15 minutes or I can not claim any overtime. I am also being told that it must require some sort of "action" besides just taking the call, for example it must require me to get on my computer or go to the site in order for it to count as overtime. So, if I get two five minute calls that wake me up in the middle of the night, I am being told I get no overtime for these calls, even if I have now lost over an hour of sleep. Can that be legal? My job description doesn't even list being on call as part of my duties, but my boss is telling me it's "just part of the job."

Would it matter what day I took the calls on? For example would it matter if I got calls after already having worked an 8 hour day versus taking the call on a weekend when I haven't worked that day but I already worked a 40 hour week?

I will not say that you have no rights, but under federal rules, your rights in such matters are not great.
- I will include the On call regulation. (http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_785/29CFR785.17.htm) This is a 60 year old rather loosely worded rule that lacks a "bright line" test. 60 years of court and administrative decisions have been mostly pro-employer. For on call pay to kick in, the employee most have their personnal time "severely restricted" as defined by the courts. While not all courts have agreed on this point, the courts idea of "severely restricted" has tended to be MUCH narrower then what most employees think.
- Actual hours worked is very different. The general rule of thumb is that all hours worked no matter where or doing what must be paid. Your employer's 15 minute rule fails this test. Saying that you must go to the computer or job site to get paid fails this test. The closest exception the employer has is the insignifcant or insubstantial test (http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_785/29CFR785.47.htm), which of course also lacks a "bright line" test.

General rules for hours worked.
http://www.dol.gov/esa/whd/regs/compliance/whdfs22.pdf