19 No answer from the user     480 Temporarily unavailable
20 Subscriber absent           480 Temporarily unavailable

I looked in the source (sofia_glue.c) and it says:

/* map sip responses to QSIG cause codes ala RFC4497 section 8.4.4 */
switch_call_cause_t sofia_glue_sip_cause_to_freeswitch(int status)
{
        switch (status) {
        ...

        ...

        case 480:
                return SWITCH_CAUSE_NO_USER_RESPONSE;