FreedomPop Starts Selling Phones, Mobile VoIP → Interesting

I already use VoIP for most of my phone calls via T-Mobile HSPA. When it works, it works well, but when data coverage sucks, so does the quality of calls.

I wonder how bad it will suck via WiMax.

How often do you experience poor call (due to poor data) quality when using VoIP via T-Mobile?

There are certain places in my area where I'm certain to get poor data coverage, not necessarily due to reception but local tower congestion. It happens from time to time.

It's much more improved ever since switching to GSM codec for all calls though.

It depends. Speex and GSM work mostly fine on a well tuned EDGE connection on T-Mobile. The wider band PCMU seems to even work at 3G or better. But be prepared to tweak a bit.