Hi,
Update to the latest version of MiCollab on your iPhone (or Android).
I know they Mitel had a bug in their last version of MiCollab Mobile that did not allow the softphone to work on LTE IPv6 networks. This was the reason why it worked on Wifi (because it was IPv4) and not on LTE. If you just noticed this happen, its because AT&T must have moved to all IPv6 on their LTE network recently. I know my wireless carrier did that and it caused all sorts of headaches with apps not supporting IPv6. (ex. OpenVPN does not work).
On the newest version, MiCollab Client works on both IPv4 and IPv6 LTE.
You do not need to update MiCollab Server but I would advise you do that as they added a bunch of new features. Please see the release notes before you do that, as you would want to upgrade all your server software at the same time (Ex. MiVB, MBG, etc).