Want to filter the caller number in Asterisk

Question

I want to filter the caller number so that i can get calls from genuine callers

Answer 1

exten => 123,1,NoOp(${CALLERID(all)})
exten => 123,n,GotoIf($[${ISNULL(${CALLERID(ani)})}=1]?NULL)
exten => 123,n,GotoIf($[${LEN(${CALLERID(num)})}<5]?NULL)
exten => 123,n,Set(isnumber=${REGEX("[0-9]" ${CALLERID(num)})})
exten => 123,n,GotoIf($["${isnumber}" = "1"]?PASS:NULL)
exten => 123,n(PASS),Dial(SIP/IP/${EXTEN})
exten => 123,n(NULL),Hangup()