I’ve created a ticket for us to look into this more and come up with a solution. In the meantime I’d recommend having a placeholder that shows the format you want or even including that requirement in the label. That might go a long way. For example:  or 
Hi Lundin, that’s a fair point. However, we don’t do phone number validation for several reasons. For one, different countries have different format standards and Umso is used by customers around the world. Even if we added an option to select a phone number format, there are challenges with validation. For example for Canada or the US it would officially be (123) 456-6789. However you’d want a user to also be able to enter 12345678 or 123 456-6789 or essentially any 10 digit number (or 11 with country code). When it comes to validation we always want to make sure we reduce friction as much as possible. We don’t want you to lose out on a potential lead because they get frustrated when inputting their phone number. It’ll be preferable to get that lead with a phone number in a wonky format. Of course this has the downside of potentially missing out on leads for which the phone number is unusable. However even with correct formatting you always run the risk of someone simply having a typo in their number. In other words, validation can do more harm than good. What we could do at best is to require it to have 10 or 11 digits but with not much more formatting than that.
@Alexis Taylor You are right, the user should be able to enter in whatever the local format is. But without knowing the country code, collecting phone numbers is very unreliable. Fortunately, this is a solved problem—just show the user a country picker. Country code + local format → validation → E164 format. This is a standard practice; there are already battle-tested libraries for doing this in the browser.  
I think length formatting would be a great minimal answer. The problem we have right now is people are leaving off area codes and we have no way to get in touch with them.