JWT,
Technically, you don't need an edge device as the MiVO-250 can do NAT Transversal on its own.
With that being said it is not ideal nor best practice to do so as the MiVO-250 doesn't support DNS SRV so the programming is non-logical at that point and therefore has to be manually updated.
If the SIP Provider has an SBC then definitely use that at it would make system programming simple.
If the SIP Provider doesn't have an SBC then I would recommend using Mitel's MBG, but in reality you should be able to use any SBC you want that works with the SIP Provider.
Thanks,
TE