WiFi Calling Falls Asleep, Inbound Echo
I have the exact same symptoms on the LG G2 I just purchased. Been trouble shooting it for a couple of weeks. At least 50% of inbound calls go to voicemail unless phone was just used. WiFi calling icon is often red for 1/2 second when waking phone up. Phone never shows missed call, no ring occurs for caller or recipient and call goes directly to voicemail as if phone is off of network. Notification of voicemail is virtually instantaneous after caller hangs up from leaving message. Also, inbound calls have bad echo for the caller. I've tried multiple networks, always the same and it only happens on LG G2. I'm also using latest release of system from LG as of 1/9. As an aside, outbound calls work reasonably well, but do occasionally just drop, forcing me to redial tech support or whoever.
T-Mobile won't acknowledge that it is a known issue even after over 20 calls. Techs won't look into the problem or respond when they say they will look into it and get back within 72 hours. They blame the router or even tell me to call LG. LG says they only make hardware and base os and that wifi calling feature is T-Mobile's to support and I agree with them.
Since I depend upon wifi calling, this phone is going back. T-Mobile has been screwing people over with this phone's wifi calling for at least 2 months. I'm out of 14 day trial period due to extensive trouble shooting so must pay for 'service' but hopefully I'll get price of phone back as I plan to return through Costco and port number back out of T-Mobile. Unlike T-Mobile, Costco typically stands behind their customers which is why I shop there.
This is a great piece of hardware, although a bit pricy at this point. Until WiFi calling works though, for me it isn't worth the price for the handset plus service.