а можно например через макро:
[гдетотам]
exten _X.,1,Macro(addprefcheck,${EXTEN})
.....
[macro-addprefcheck]
exten => s,1,ExecifTime(00:00-23:59,*,*,*?GotoIF($["${LEN(${ARG1})}" = "4" ]?check))
..........
same => n(check),ExecIf($["${ARG1:0:1}" = "6" & "${ARG1:1:1}" > "4" & "${ARG1:1:1}" < "6 ...