Thanks for the follow up. The SIP trunk is provided by VoicePulse. I have discovered, in extensions_additional.conf, the destination I defined for the DAHDI trunk [ext-did-0001] and the destination I defined for the SIP trunk [ext-did-0002]. These each are listed in [ext-did] in sequence such that regardless of which trunk a call comes in, [ext-did] always takes [ext-did-0001]!
[ext-did]
include => ext-did-custom
include => ext-did-0001
include => ext-did-0002
exten => foo,1,Noop(bar)
;--== end of [ext-did] ==--;