Extending UM91214B to support more channels
The four outputs actually give a binary code out, so there are 16 possible combinations. This circuit only uses the codes where only 1 but is high:   

0001  
0010  
0100  
1000  


If you would use one of the other codes you would switch 2 or more outputs simultaneously.  

What you want is a 4-to-16 demultiplexer, like the 74HC154. This will give you a low pulse on one of the 16 outputs when the corresponding DTMF code is sent. The original schematic gives an active high pulse. This means that either you have to invert the polarities of all the outputs, or that the output will only switch when the key is released instead of being pressed.  

For the transmitter you simply have to add extra buttons in the rows/columns matrix.