There are no real standards for DTMF (touchtone) key meanings
Although there are no legal or strong industry standards to rely upon for the assignment of actions to DTMF keys, there are some common practices.

0 for Operator (Agent, Customer Service Rep) in the United States
For example, in the US, it is virtually inconceivable to use a single press of 0 for anything other than connecting to an operator (customer service representative). This doesn't mean that you must design your system to immediately connect to a human, but your system should handle this when it happens (also see Zero-Outs).

International differences
Be aware that practices in the United States are not universal worldwide. For example, in India, callers who want assistance will press 9 rather than 0. For international design, ensure that someone is available who can review your plans for DTMF usage.

Be consistent within the IVR
It is important to be consistent within a given application. It's common to use pound (#) and star (*) for global commands because they aren't usually used in any other type of entry. A recommended usage is # to skip or repeat, and * return to previous menu or to cancel input (Lewis, 2011). Other usage observed is # for main menu and star for repeat. Whatever you do, however, be consistent to as great an extent as possible, within the IVR, within the brand, within the company, and within the industry.

Using pound as an input terminator
The purpose of using the pound key as a terminator is so callers who are making a variable length entry (5, 7, or 10 digit account numbers, for example) don't have to wait for the incomplete timeout to trigger. The silence is uncomfortable and sometimes makes the caller think she did something wrong. By allowing the caller to enter the account number, followed by pound, that awkward silence goes away because pound indicates to the system that the caller has completed the entry.

There are a couple of issues with using pound as a terminating character. One is, from the caller's perspective, she never knows if she's supposed to press pound after an entry or not. Every IVR out there isn't consistent, so it keeps the caller guessing what she should do. The best thing to do is to always accept pound, even if the IVR doesn't prompt the caller to enter it. It's foolish to reject an entry because pound was provided. The IVR is smart enough to recognize it and move on. The other issue is many callers speak their response instead of entering. Using pound only makes sense for DTMF entry, so if a caller speaks an utterance that is a variable length, the uncomfortable silence may be unavoidably present.

nb: In the UK # is referred to as 'hash'