Thank you! I should have paid closer attention to that.
I'm still getting the following and can't seem to find the issue ...
Attempt to reach a non-existent destination for Gosub(Context:Main-IVR, Extension: s, Priority:1)
extensions_custom.conf looks like this now ... it's the only thing in the extension_custom.conf file.
[Main-IVR]
exten => s,1,SayDigits(8675309)
exten => s,n, Return()