Fixing CDR records for anonymous calls with Asterisk

Yesterday I was faced with an Asterisk problem logging src CDR records as anonymous, when dialing anonymously from a Snom SIP phone. In this particular situation it was OK to use anonymous as callerid, but not in the CDR records.

Using something like Set(CDR(src)=12345678) doesn't work since most of the CDR records are read-only. Also using Set(CALLERID(num)=12345678) also doesn't give you the expected result for the src CDR record.

However if you set the ANI callerid you will get the expected result, like this: Set(CALLERID(ANI)=12345678).