I am new to Switchvox. My company has a location we acquired that uses Digium Switchvox SMB Edition 4.0 for PBX. They have a asterisk gateway to take calls from PSTN and pass to the Switchvox. In extensions.conf the DID routing is set up to send calls according to DNIS coming in. When calls are handed off to the Switchvox all calls go to extension 800 which is the main auto attendant. No matter what The provider they set up has 800 as the callback extension. How can I make the Switchvox route calls to DID. All the DID are set up in the Switchvox incomming calls and the provider is set to yes apply incoming call rules. I can not leave the callback extension blank. System says it is required.

Can you please explain further.

If you have POTS lines coming in, you are somewhat limited as to how you can route the calls.

If however you have DID lines, then you should be able to route the caller based on what number they dialed.  Example:
If you have 404-555-1000 thru 1099, then based on the telephone number your customer dialed 1000 thru 1099, you should be able to route the caller to which over extension or service you desire.
It is important to note that the service provider can be sending any number of digits.  Typically they send 10, 7, 4, or 3.  YOu would build your incoming call routes accordingly.