We needed to route all tenants based on the SIP TO: header. I threw this into extensions.conf (one-liner found on voip-info.org):
[from-outside-redir]
;No DID? Set it according to SIP To: header and re-route
exten => s,1,Goto(from-outside,${CUT(CUT(SIP_HEADER(To),@,1),:,2)},1)
#include.....