New function for library

I want to add a function to return the number of bytes in the RX buffer of the A110x2500Radio library file. I added the function to A110x2500Radio.cpp and definition to A110x2500Radio.h. But I get an error when I compile it into the application. Is there another step I am missing? I will want to create some other functions built on the library.

WiFiXivelyClient.cpp.o: In function `loop':
WiFiXivelyClient.cpp:(.text.loop+0x42): undefined reference to `A110x2500Radio::getNumBytes()'
collect2.exe: error: ld returned 1 exit status

thanks,

Bill