Outbound call "Request Entity Too Large"

Using the SIPml API and a self hosted instance of webrtc2sip I am able to make incoming calls fine, but when I attempt to make an outgoing call I get a terminated event with 413 Request Entity Too Large.

Initiating outgoing call:

this.currentCall = this.sipStack.newSession('call-audio', {
  audio_remote: new Audio(),
  events_listener: { events: '*', listener: this.eventsListener.bind(this) },
});
this.currentCall.call('+441473650481');

Sequence of SIP events:

SEND: INVITE sip:+441473650481@sip.uk-rdl.btone.bt.com SIP/2.0
Via: SIP/2.0/WSS df7jal23ls0d.invalid;branch=z9hG4bKOTDHplMmyNZlGPpWQPtzUeX9vzVAymO4;rport
From: <sip:ova21243002@sip.uk-rdl.btone.bt.com>;tag=RAt8tHp0HwoxwmCnU7gp
To: <sip:+441473650481@sip.uk-rdl.btone.bt.com>
Contact: "undefined"<sips:ova21243002@df7jal23ls0d.invalid;rtcweb-breaker=yes;click2call=no;transport=wss>;impi=ova21243002;ha1=0854005f14636db60a73dad82c09fabc
Call-ID: 7fb88945-5fa4-ea10-0078-d0ea60fc5196
CSeq: 5014 INVITE
Content-Type: application/sdp
Content-Length: 1897
Route: <sip:sip.uk-rdl.btone.bt.com:5062;lr;sipml5-outbound;transport=udp>
Max-Forwards: 70

recv=SIP/2.0 100 Trying (sent from the Transaction Layer)
Via: SIP/2.0/WSS df7jal23ls0d.invalid;rport;branch=z9hG4bKOTDHplMmyNZlGPpWQPtzUeX9vzVAymO4
From: <sip:ova21243002@sip.uk-rdl.btone.bt.com>;tag=RAt8tHp0HwoxwmCnU7gp
To: <sip:+441473650481@sip.uk-rdl.btone.bt.com>
Call-ID: 7fb88945-5fa4-ea10-0078-d0ea60fc5196
CSeq: 5014 INVITE
Content-Length: 0

recv=SIP/2.0 180 Ringing
Via: SIP/2.0/WSS df7jal23ls0d.invalid;rport;branch=z9hG4bKOTDHplMmyNZlGPpWQPtzUeX9vzVAymO4
From: <sip:ova21243002@sip.uk-rdl.btone.bt.com>;tag=RAt8tHp0HwoxwmCnU7gp
To: <sip:+441473650481@sip.uk-rdl.btone.bt.com>;tag=1429840597
Contact: <sips:+441473650481@192.168.1.144:10062;transport=wss;ws-src-ip=81.151.237.142;ws-src-port=1024;ws-src-proto=wss>
Call-ID: 7fb88945-5fa4-ea10-0078-d0ea60fc5196
CSeq: 5014 INVITE
Content-Length: 0
Allow: ACK,BYE,CANCEL,INVITE,MESSAGE,NOTIFY,OPTIONS,PRACK,REFER,UPDATE

recv=SIP/2.0 413 Request Entity Too Large
Via: SIP/2.0/WSS df7jal23ls0d.invalid;rport;branch=z9hG4bKOTDHplMmyNZlGPpWQPtzUeX9vzVAymO4
From: <sip:ova21243002@sip.uk-rdl.btone.bt.com>;tag=RAt8tHp0HwoxwmCnU7gp
To: <sip:+441473650481@sip.uk-rdl.btone.bt.com>;tag=1429840597
Call-ID: 7fb88945-5fa4-ea10-0078-d0ea60fc5196
CSeq: 5014 INVITE
Content-Length: 0
Reason: text="Request Entity Too Large";cause=413;text="Request Entity Too Large"

SEND: ACK sip:+441473650481@sip.uk-rdl.btone.bt.com SIP/2.0
Via: SIP/2.0/WSS df7jal23ls0d.invalid;branch=z9hG4bKOTDHplMmyNZlGPpWQPtzUeX9vzVAymO4;rport
From: <sip:ova21243002@sip.uk-rdl.btone.bt.com>;tag=RAt8tHp0HwoxwmCnU7gp
To: <sip:+441473650481@sip.uk-rdl.btone.bt.com>;tag=1429840597
Call-ID: 7fb88945-5fa4-ea10-0078-d0ea60fc5196
CSeq: 5014 ACK
Content-Length: 0
Route: <sip:sip.uk-rdl.btone.bt.com:5062;lr;sipml5-outbound;transport=udp>
Max-Forwards: 70

DoubangoTelecom / sipml5

Outbound call "Request Entity Too Large" #267