Even in government contract situations in which per diem is built into the contract, the company isn't obligated to give it to the employees; that's what the government gives the company. When not in such a contract, the company still has zero legal obligation to provide you with per diem. They should, but shoulda, coulda, woulda. When home...most operators don't pay pilots per diem. I don't know any home-base operators that pay perdiem while the employee is at home, regardless of whether the employee is on call.
If you are on call, it does not necessarily mean you are on duty. It does mean you're not at rest, if you have an obligation to act for the company; it does not mean that your duty has started. The FAA puts a fine point on this in numerous legal interpretations. Your limiting factor under 135 is the ability to look back and find rest; if you're not getting rest on call, while you're not on duty, you're still limited by what duty you can provide once you're called.
Regardless of on duty or on call, the company isn't required to provide you with perdiem: neither a given amount, nor perdiem at all.
If you have no union, then there's no legal duty to meet, that establisehs a perdiem requirement. If the company has provided an employee handbook, then the company is obliged, but not obligated, to adhere to their own policy. Remember, it's the company's policy, not yours, and the company may set what rules it wishes, so long as it's legal.
If you happen to have per diem spelled out in a FAA-approved document (eg, general operations manual), which would be rare, then in that case, the company would be obligated to adhere to the policies set forth in the manual, and has a legal duty to do so.