RJ11 socket in Eagle libraries?
Sparkfun releases many of the parts used in their products in the form of the footprint and schematic symbol.  It seems like they have an RJ11 connector in their connections library.  You can download them from the linked github page.  

As for more Eagle library resources:


Adafruit also releases their eagle parts as well, although I'm not sure if they have an RJ11 connector.
I have published and maintain my own as well, although I take great care to make sure they are correct, they are still unverified so take them w/ a grain of salt.


Generally if you can't find the part, you'd have to make it yourself.  There are many tutorials explaining how to make a part in Eagle.