fix: use PAI sip: identity for auto-computed Referred-By header

When referredBy is not explicitly provided, the code was using
callingNumber (user part only) combined with the SBC's internal IP
from dlg.remote.uri. This produced a Referred-By with an IP address
(e.g. sip:+number@10.x.x.x) that carriers like Verizon reject.

Instead, extract the full sip: name-addr from the P-Asserted-Identity
header, which preserves both the display name and the original FQDN
(e.g. "Cha Van" <sip:+number@ims.lte.wal.verizon.com>). Falls back
to the previous behavior when PAI is not available.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/tasks/sip_refer.js

@@ -122,8 +122,20 @@ class TaskSipRefer extends Task {
     }
     else this.referToIsUri = true;
     if (!referredBy) {
-      /* default */
-      referredBy = cs.req?.callingNumber || dlg.local.uri;
+      /* default: extract the sip: identity from PAI, preserving the original domain */
+      const paiHeader = cs.req?.has('p-asserted-identity') ? cs.req.get('p-asserted-identity') : null;
+      if (paiHeader) {
+        const parts = paiHeader.split(/,\s*(?=")/);
+        for (const part of parts) {
+          if (/<sips?:/.test(part)) {
+            referredBy = part.trim();
+            break;
+          }
+        }
+      }
+      if (!referredBy) {
+        referredBy = cs.req?.callingNumber || dlg.local.uri;
+      }
       this.logger.info({referredBy}, 'setting referredby');
     }
     if (!referredByDisplayName) {