Also... It doesn't matter if they stay... VERIZON is going to have VoLTE Live by the end of summer on the ENTIRE NETWORK, and slowly start releasing VoLTE (NON CDMA) Phones out by Years end.... Sunsetting CDMA by 2021 or earlier depending on how fast customers upgrade to new phones!!!
SPRINT's New Network is also VoLTE as well as CDMA and is talking of turning on their VoLTE in Summer 2015, and again releasing VoLTE Only devices by end of 2015
AT&T and TMOBILE are also in the process of prepping Their towers to ALL VoLTE...
The Upside to ATT/TMOBILE and all GSM carriers is they will have HSPA+ 3G GSM as a back up.... because aVoLTE Voice Call can not transfer to CDMA back up if needed!
Verizon and Sprint had to upgrade EVERY SINGLE TOWER in the company to 4G VoLTE because of Not being able transfer Voice Calls between VoLTE and CDMA...
Once Verizon and SPRINT turn on VoLTE only devices.... 4G LTE will be the slowest on a Verizon or Sprint VoLTE Only Device