Where I work, we use Micollab softphone application installed on a regular mobile phone. But, I'm not a fan, I find it laggy and very susceptible to network drops. Sure, someone may say you need good wifi infrastructure to make it work and it's true. We do extensive coverage, some 2000 APs in total, and handovers from access points work fine for pretty much any other application the same mobile phone. Mitel app though...