GSM (Global System for Mobiles) and CDMA (Code Division Multiple Access) are very different standards. Phones designed for one will not work with the other although there are some Android phones that now support both.
GSM is the most widely used in the world.
Verizon uses CDMA and T-Mobile uses GSM. A T-Mobile phone will only work with T-Mobile service.
T-Mobile is one service offers WiFi calling. You will need a GSM phone, your current CDMA iPhone will not work on T-Mobile.
Almost all phones and services can be used for WiFi calling with an app like, Vonage (requires Vonage VOIP service), Skype, Google Voice, etc. but to use wireless calling with your carrier's service is dependent on them and your particular phone.
A phone must support 4G LTE for it to work; a sim can't change your phone if it doesn't support it.
A "hotspot" which enables you to set up your phone to give wifi connections using your phone data to any computer or other device in the same way a wireless router does. This is part of operating system on your phone and is supported by most all carriers.
An unlocked phone can be used with any service that uses the same standard (GSM or CDMA) the phone supports. A locked phone can only be used with the carrier it is locked to; a Verizon phone only with Verizon, an AT&T phone only with AT&T, etc.
