The Pixel 4 probably reliably called 911 on the simplest network config, like GSM or CDMA, but when encountering WCDMA, VoLTE and all the fun ways you can configure these technologies, you ended up with situations where the phone would perhaps have data service, but no ability to dial 911.
T-Mobile has had manufacturers brick band 12 on phones like the Moto E because you would get rural data coverage with this band, but T-Mobile did not sell any Moto phones and did not want to write a software config for VoLTE for Motorola phones. Without band 12 enabled the phone will happily roam onto AT&T or Verizon in areas where T-Mobile doesn't offer GSM service instead of hanging out on a cellular network that can never provide calling or texting at that location.
T-Mobile has had manufacturers brick band 12 on phones like the Moto E because you would get rural data coverage with this band, but T-Mobile did not sell any Moto phones and did not want to write a software config for VoLTE for Motorola phones. Without band 12 enabled the phone will happily roam onto AT&T or Verizon in areas where T-Mobile doesn't offer GSM service instead of hanging out on a cellular network that can never provide calling or texting at that location.