SIP/2.0 503 Service Unavailable
<allOneLine>
Via: SIP/2.0/TCP host129.example.com
;branch=z9hG4bKzzxdiwo34sw
;received=192.0.2.129
</allOneLine>
To: <sip:user@example.com>
From: <sip:other@example.net>;tag=2easdjfejw
CSeq: 9292394834772304023312 OPTIONS
Call-ID: scalarlg.noase0of0234hn2qofoaf0232aewf2394r
Retry-After: 949302838503028349304023988
Warning: 1812 overture "In Progress"
Content-Length: 0
