exten => h,n,ExecIf($[${DIALSTATUS} = ANSWER],Set,TIME_USED=${MATH(${ANSWEREDTIME}/60,f)})

    -- Executing [h@subPlaceCall:2] Verbose("SIP/4918847473-1565e170", "1|DIALSTATUS = CANCEL") in new stack
 DIALSTATUS = CANCEL
[Feb  8 12:21:51] WARNING[19032]: func_math.c:169 math: '' is not a valid number
    -- Executing [h@subPlaceCall:3] ExecIf("SIP/4918847473-1565e170", "0|Set|TIME_USED=") in new stack