SIM800L rarely responds to commands
Making an answer from my comments:

I think you have a couple of small problems:


The LM317 requires a minimum difference of 3 volts between the input and output. 5-3=2. Your regulator isn't regulating. 
Run the calculations for the output voltage as given in the datasheet. I think you will find that your resistor values are wrong.


You need a powersupply with a higher output than you have.  To get 3.8V out, you need at least 6.8V in.

The correct values for the resistors can be calculated using the suggested circuit and formula given in the datasheet.

Circuit:



Formula:



Rewritten to solve for R1 when given R2 and the desired output voltage:

$$ R_1 = \frac{R_2}{\frac{V_o - I_{adj}R_2}{V_{ref}}-1}$$

Where:


\$V_{ref} = 1.25V\$
\$I_{adj} = 50 \mu A \$


Given \$R_2 = 2000 ohms\$ and \$V_o = 3.8V\$, I get R1 as 1020 ohms.  My "eyeball estimate" of the resistors was wrong, and your original values were pretty much spot on.

Really all you needed was enough input voltage for the regulator to work.