My husband and I both have an iPhone 4s and the problem occurs on both phones. The MicroCell used to work fine, and appears to continue to work fine (the phones appear to connect to the MicroCell, all of the lights on the front of the MicroCell are lit and green), but people we call or who call us cannot hear us after the first few seconds of each call. We can hear the other people just fine but "outbound transmission" does not work. I have tried calling businesses needing keypad entered information or voice recognition and those are not "transmitted" either.
This is frustrating because we dropped the landline in favor of the cell phones, but now basically have no phone service.
We have AT&T Uverse internet with a Pace 5031NV (AT&T supplied) modem/router.
Call quality problems are usually caused by latency, jitter, packet loss, and bandwidth issues and can really only be fixed by the ISP. If all of your lights are solid green (power, ethernet, GPS, and 3G), and you can receive calls ok, then the MicroCell is working as designed.
You said that call quality used to be fine. Did it deteriorate once you got the U-verse Pace 5031 RG? Being as you have the Pace, that indicates that you have VDSL2, single bonded pair, and no IPTV. Don't expect the U-verse support people to help you with the MicroCell and vice versa, there is very little communication between the two support groups and neither one understands how the other service works.
There are apps available that allow your to run latency, jitter, packet loss tests but those are just "snap-shots" for a given time window. They really need to be run and monitored over a period of time to get an accurate picture, something that AT&T can, and should do for you.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.