How secure is T-mobile's service? Is it encrypted?

How secure is T-mobile's service? Is packet sniffing or a man in the middle attack possible on the T-mobile network?

I know that most ISPs have a good idea of what a user does on their network. You can typically just use a VPN to enable more anonymity online. But from the signal between a device and the cell tower i'm guessing there can be multiple security issues.

Lets say I login to my T-mobile account or bank account. And someone has set up a repeater or fake signal that my phone connects to thinking its the T-mobile service. Would that person be able to see the data being transferred over the network. Or is the signal my phone sends out encrypted not allowing for this to happen.

Devices like the 4G LTE Cellspot connects to a customers own router and allows them to broadcast a T-mobile signal using their home internet. Are people that own this device able to see data and traffic from my phone? They could possible configure their router maliciously to look at all traffic coming from a 4G LTE Cellspot device with unsuspecting T-mobile customer connecting to it thinking its connected directly to a cell tower.

Another way this device can be abused is the person can limit their bandwidth to the device to unusable speeds but all phones around it will still connect to it because its the strongest T-mobile signal. This would degrade the service and make the T-mobile service around that device practically unusable.

I swear I'm not paranoid. Just a student studying security and these questions came to mind.

Gracias.