Cellular Modules Weird Voltage 3.8V
You can connect those modules directly to a lithium ion battery. They are specified for 3.2 V to 4.2 V (at least the Huawei MC50 is).

The nominal voltage of a single lithium cell is given between 3.6 V and 3.8 V depending on the exact chemistry. If it stops working below 3.2 V it even provides a form of battery protection as it doesn't drain the battery to dangerously low levels (dangerous for battery life, no explosions).

So you can save the power management circuitry. If you have a 3.3V power supply in your application, you can power the module with that as well.