You're on the right track — here's the concise plan and exactly what to do. 1) Map the SIP header to an attribute • Add

headers_to_attributes

on your inbound trunk:

{
  "trunk": {
    "name": "My Inbound Trunk",
    "numbers": [
      "+15105550100"
    ],
    "headers_to_attributes": {
      "X-TCN-Virtual-Number": "virtualNumber"
    }
  }
}

This exposes the header value as participant attribute

virtualNumber

. 2) Add the +1 prefix in your app where you read the attribute • LiveKit does not auto-format phone numbers, so format after retrieving

virtualNumber

. Example functions: Node.js:

function formatVirtualNumber(v) {
  if (!v) return v;
  if (v.startsWith('+')) return v;
  if (v.length === 10) return '+1' + v;
  if (v.length === 11 && v.startsWith('1')) return '+' + v;
  return v;
}

Python:

def format_virtual_number(v):
    if not v: return v
    if v.startswith('+'): return v
    if len(v) == 10: return '+1' + v
    if len(v) == 11 and v.startswith('1'): return '+' + v
    return v

Use the formatted value (e.g.,

+17788071156

) in your app logic after retrieving

participant.attributes['virtualNumber']

. Would you like a version that enforces E.164 validation or handles other country codes? Sources: SIP APIs | LiveKit docs | sip | livekit/livekit_composite | DeepWiki