Help Fix ER101 Invalid Certificate message?

[km2002]

Hi, folks. Running a Galaxy S5 on MetroPCS. Two days ago it started throwing this ER101 Invalid Certificate message in Wi-Fi Calling mode. I've tried more than a few things to correct it. Turned Wi-Fi Calling off and on; no change. Turned Wi-Fi off and on; no change. Even though a lot of posts across the web say it won't work, they also say that TS will tell you to turn off the phone, pull out the SIM card, reinsert it, restart the phone. Well, I did that: No luck. In another place it was suggested that to solve this on MetroPCS, create an APN for T-Mobile; no happy there, either. I've also read that one needs to get the network provider involved, instigate something called a network inquiry. I've gotten this error message on networks provided by at least three different ISPs; so I don't think it's a network issue. It's acting like a phone issue in this case; and it prevents me from calling over Wi-Fi. Never had a problem going from net to net, ISP to ISP — until about two days ago; now I get this error everywhere. Anyone have a reliable way to fix this once and for all on all networks and all ISPs? TIA.