Virtual Private Networking (VPN) is a secure (encrypted) way of
connecting to private resources (business LAN, home computer) over the public
Internet. To make a VPN connection, VPN client software typically connects to VPN server software.
VPN types include Point To Point Tunneling Protocol (PPTP, deprecated but still
useful) and OpenVPN (recommended,
secure). But you may have a problem making VPN connections over
T-Mobile USA wireless data.
The Symptoms: You are
able to do things on your mobile device over T-Mobile data (browse the
Internet, send and receive email, etc), and you are able to make VPN connections
over Wi-Fi wireless, but you are not able to make VPN connections over T-Mobile
wireless data.
The Cause: T-Mobile apparently
pushed out a data configuration in late 2013 that set APN Protocol
to IPv6. That works properly for most things on mobile devices,
but it prevents PPTP (built into Android) and OpenVPN (OpenVPN
Connect) clients from making VPN connections.
The Cure: Open Settings > Wireless & Networks > More... > Mobile networks > Access Point Names > T-Mobile GPRS (fast.t-mobile.com)
> APN protocol, and change the selection from IPv6 to IPv4/IPv6 IPv4 (see update below).
The Caveat: This cure was tested
successfully with a Nexus 5 running Android 4.4.2 KitKat on a Prepaid plan, and it might not work properly on other devices or plans. Use
at your own risk.
See Also: Secure Wi-Fi with VPN
Update: IPv4/IPv6 currently causes issues with some websites (example), so try IPv4.