Voice Call fading in & out after gingerbread update.. anyone else?

Tmobile said the NEW wifi voice needed a new sim card to work properly.

It improved all my issues. WIfi voice working great.

Make sure you have your home router set to following settings for best voice quality:
Wireless QoS enabled (Quality of Service): Enabled
WWM:Enable
wireless No Acknowledgement: Enabled (you don't want the router to acknowledge or check for minor mistakes on wifi transmits--speeds it up dramatically).

Newer routers should all have QOS settings in them (gamers use them). QOS forces data to always sorta LINE up info so nothing is fragmented. Voices canNOT be sent in fragments (or you get stutter). LIKE quality online live video games or video chats can't be fragmented.
If your router doesn't have QOS, I'd, honestly, buy a newer router. Video, voice, games, multimedia all take advantage of QOS.

Wifi voice UMA settings