Most growth hacks are inappropriate. It's not a risk-free endeavor. It hedges on whether the customer would actually be annoyed enough to stop using the service in disgust, which is uncommon. (of course, hacking a voicemail would have expected consequences)

I'd push back on that first part. I think there are plenty of appropriate marketing tactics you could put under the growth hack umbrella.

To me, things are inappropriate when:

1. They're blatantly illegal 2. They cause damage or mental anguish to another (revenge, gossip, etc.)

Even with both of those though we're looking at some grayness, especially with measuring #2.

I do like the measuring stick of: would individuals be annoyed/disgusted/pissed enough to stop using.

If you ask the HN crowd, effectively all growth hacks will be considered spam/inappropriate. Case in point:

https://news.ycombinator.com/item?id=8455138

In general, tech people believe that any message of any kind that may result in revenue, clicks, or new users for the sender is spam. I think real users consider unsolicited messages that don't deliver value to them to be spam, and welcome things that do deliver some value to them.

Most "growth hacks" are not "hacks" in the malicious sense but rather hacks in the sense of doing something clever. (see "life hack", etc.) This clearly crosses that line and even though they claim it wasn't malicious, it's an invasion of someone's personal space. I'd expect this to backfire massively, it shows a tremendous lack of judgment.

If you have to question it then it's probably inappropriate. That's how I look at it personally.

Wow, I'd think it would be illegal to change someone's voicemail greeting.