A number of providers (mostly low cost and free ones) don't support DID routing because they don't really advertise support for Asterisk. It would be extremely helpful to be able to route these incoming calls to various contexts or extensions based upon the channel ID. For example, here's the way we did way back in the AMP days:
[all-incoming]
exten => s,1,NoOp?(Default incoming SIP call path activated for ${CHANNEL})
exten => s,2,GotoIf?($[${CHANNEL:0:14} = SIP/67812334567]?custom-broadvoice-in,6781234567,1)
exten => s,3,GotoIf?($[${CHANNEL:0:12} = SIP/yourname]?custom-axvoice-in,8431234567,1)
exten => s,4,GotoIf?($[${CHANNEL:0:8} = SIP/6234]?custom-stanaphone-in,3471234567,1)
exten => s,5,GotoIf?($[${CHANNEL:0:17} = SIP/67.43.155.130]?custom-callpacket-in,7071234567,1)
exten => s,6,GotoIf?($[${CHANNEL:0:14} = SIP/5151234567]?custom-freedigits-in,5151234567,1)
exten => s,7,Goto(from-internal-custom,99,1)