POST /v1/phone-numbers/{phone_id}/link

curl -X POST https://api.vonative.com/v1/phone-numbers/pn-abc123-.../link \
  -H "Authorization: Bearer YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "assistant_id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890"
  }'

{
  "success": true,
  "data": {
    "phone_id": "pn-abc123-...",
    "assistant_id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
    "endpoint_id": "ep_abc123def456",
    "sip_uri": "sip:ep_abc123def456@sip.krosai.com"
  },
  "message": "Phone linked to assistant successfully"
}