diff -r -u asterisk-1.4.13/channels/chan_sip.c asterisk-1.4.13-sipprefix/channels/chan_sip.c
--- asterisk-1.4.13/channels/chan_sip.c 2007-10-10 15:42:00.000000000 +0100
+++ asterisk-1.4.13-sipprefix/channels/chan_sip.c       2007-11-09 22:48:32.000000000 +0000
@@ -1485,7 +1485,7 @@
 static int create_addr_from_peer(struct sip_pvt *r, struct sip_peer *peer);
 static int create_addr(struct sip_pvt *dialog, const char *opeer);
 static char *generate_random_string(char *buf, size_t size);
-static void build_callid_pvt(struct sip_pvt *pvt);
+static void build_callid_pvt(struct sip_pvt *pvt, const char *prefix);
 static void build_callid_registry(struct sip_registry *reg, struct in_addr ourip, const char *fromdomain);
 static void make_our_tag(char *tagbuf, size_t len);
 static int add_header(struct sip_request *req, const char *var, const char *value);
@@ -2897,6 +2897,7 @@
        struct varshead *headp;
        struct ast_var_t *current;
        const char *referer = NULL;   /* SIP refererer */
+       const char *callidprefix = NULL;
 
        p = ast->tech_pvt;
        if ((ast->_state != AST_STATE_DOWN) && (ast->_state != AST_STATE_RESERVED)) {
@@ -2904,6 +2905,14 @@
                return -1;
        }
 
+       callidprefix = pbx_builtin_getvar_helper(p->owner, "SIP_CALLID_PREFIX");
+       if (callidprefix) {
+               /* Rebuild the callid with the prefix */
+               ast_log(LOG_NOTICE, "Setting SIP Call-ID prefix to %s\n", callidprefix);
+
+               build_callid_pvt(p, callidprefix);
+       }
+
        /* Check whether there is vxml_url, distinctive ring variables */
        headp=&ast->varshead;
        AST_LIST_TRAVERSE(headp,current,entries) {
@@ -4304,13 +4313,16 @@
 }
 
 /*! \brief Build SIP Call-ID value for a non-REGISTER transaction */
-static void build_callid_pvt(struct sip_pvt *pvt)
+static void build_callid_pvt(struct sip_pvt *pvt, const char *prefix)
 {
        char buf[33];
 
        const char *host = S_OR(pvt->fromdomain, ast_inet_ntoa(pvt->ourip));
 
-       ast_string_field_build(pvt, callid, "%s@%s", generate_random_string(buf, sizeof(buf)), host);
+       if (prefix)
+               ast_string_field_build(pvt, callid, "%s-%s@%s", prefix, generate_random_string(buf, sizeof(buf)), host);
+       else
+               ast_string_field_build(pvt, callid, "%s@%s", generate_random_string(buf, sizeof(buf)), host);
 
 }
 
@@ -4421,7 +4433,7 @@
                ast_string_field_set(p, fromdomain, default_fromdomain);
        build_via(p);
        if (!callid)
-               build_callid_pvt(p);
+               build_callid_pvt(p, NULL);
        else
                ast_string_field_set(p, callid, callid);
        /* Assign default music on hold class */
@@ -11149,7 +11161,7 @@
                if (ast_sip_ouraddrfor(&p->sa.sin_addr, &p->ourip))
                        p->ourip = __ourip;
                build_via(p);
-               build_callid_pvt(p);
+               build_callid_pvt(p, NULL);
                ast_cli(fd, "Sending NOTIFY of type '%s' to '%s'\n", argv[2], argv[i]);
                transmit_sip_request(p, &req);
                sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
@@ -15234,7 +15246,7 @@
                if (ast_sip_ouraddrfor(&p->sa.sin_addr, &p->ourip))
                        p->ourip = __ourip;
                build_via(p);
-               build_callid_pvt(p);
+               build_callid_pvt(p, NULL);
                /* Destroy this session after 32 secs */
                sip_scheddestroy(p, DEFAULT_TRANS_TIMEOUT);
        }
@@ -15516,7 +15528,7 @@
        if (ast_sip_ouraddrfor(&p->sa.sin_addr, &p->ourip))
                p->ourip = __ourip;
        build_via(p);
-       build_callid_pvt(p);
+       build_callid_pvt(p, NULL);
 
        if (peer->pokeexpire > -1)
                ast_sched_del(sched, peer->pokeexpire);
@@ -15701,7 +15713,7 @@
        if (ast_sip_ouraddrfor(&p->sa.sin_addr, &p->ourip))
                p->ourip = __ourip;
        build_via(p);
-       build_callid_pvt(p);
+       build_callid_pvt(p, NULL);
 
        /* We have an extension to call, don't use the full contact here */
        /* This to enable dialing registered peers with extension dialling,