If we are going to ignore the fact the Mitel cannot do or support NAT translation and hope the SIP ALG of the firewall handles it, I highly suggest Bria Solo softphone... We use it with Mitel, ESI, and Broadsoft hosted platforms and it works very well. You can try it for free, but it is not free for normal use but is well worth the $30/year cost as it stores all it's config in the cloud... All the user needs to know for any platform (mobile, PC, tablet, etc) is their login to the Bria website and the rest is handled automatically.