You are using a network-style (non-inverting) RJ11 cable. The Handy Board must be used with telephone-style (inverting) RJ11 cables.
If you use a network style cable, you invert the power and ground connections, which overheats the 47 ohm charge resistor (R11) on your serial interface board, along with potential for greater damage.
Here is how you tell if you have a telephone-style or network-style RJ11 cable. Hold the two ends together, metal contact side up, as if you were trying to plug them into each other.
Then look at the wiring colors. On a telephone cable, each color should line up with itself on the other end.
On a network cable, the colors are inverted.
See the diagram below for clarification. Again, please, please, PLEASE! be careful about this and do not use network cables with your Handy Board.