This is a problem that has been solved many times, and in many ways, so I wouldnt try to reinvent the wheel, this site:
has a webservice to perform the action that you need - if your servers are internet connected
btw - 20/25 years ago, I worked on a project that used their product, so they have been around for a while.
One thing you would need to consider is that US numbers generally drop the 'and' between hundreds/thousands and the remainder of the number - you will have to test and insert it to keep it 'proper'
@cassymfreeman, I've never had a need to implement anything like this in nintex world.
and, afaik, nintex doesn't provide anything like that.
but I know ORACLE database provides resp. can be tweaked to provide such a function.
if you have one reachable, with a SQL request control that should be pretty simple either in forms or workflow