Adding code to from-sip-external using extensions_custom.con

Hello All,

 I was finding that there's a long delay from when the call is picked up to when the attendant answers, so I added a new s,1 line to [from-sip-external] in extensions.conf so that the caller continues to hear ringing until the TrixBox plays back the attendant.  Here's the line of code:

exten => s,1,Playtones(ring)

 This works great, but I understand that, when changing settings using FreePBX, this line of code may be over-written.

 Can someone please explain how to maintain this new functionality without adding it directly to extensions.conf?  I assume the correct place to do it is in extensions_custom.conf.  Do I create a [from-external-custom] context and just have the line of code above in it?  Do I then have to GoTo to [from-sip-external]?  I'm a bit lost at this point.

 Thanks for any help you can provide,