healthcheck API always returns 400 status

Always get a 400 Status from healthcheck API.

Any clues as to how this can happen?

Getting auth token always works OK, bearer token is valid.

CURL Verbose output:

* Connection #0 to host tapi.telstra.com left intact
*   Trying 13.210.2.39:443...
* Connected to tapi.telstra.com (13.210.2.39) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*  CAfile: C:\Program Files\PHP\curl-ca-bundle.crt
*  CApath: none
* SSL connection using TLSv1.2 / ECDHE-RSA-AES256-SHA384
* ALPN, server accepted to use http/1.1
* Server certificate:
*  subject: C=AU; ST=Victoria; L=Melbourne; O=Telstra Corporation Limited; CN=tapi.telstra.com
*  start date: Jul 29 02:29:03 2021 GMT
*  expire date: Aug 26 00:00:00 2022 GMT
*  subjectAltName: host "tapi.telstra.com" matched cert's "tapi.telstra.com"
*  issuer: C=BM; O=QuoVadis Limited; CN=QuoVadis Global SSL ICA G3
*  SSL certificate verify ok.
> GET /v2/messages/sms/healthcheck HTTP/1.1
Host: tapi.telstra.com
Accept: */*
Content-Type: application/json
Authorization: Bearer xxxx
Content-Length: 0

* Mark bundle as not supporting multiuse
< HTTP/1.1 400 Bad Request
< Date: Thu, 06 Jan 2022 22:43:55 GMT
< Content-Type: text/html
< Content-Length: 150
< Connection: keep-alive
< Server: nginx
< Strict-Transport-Security: max-age=15768000; includeSubDomains
<
* Connection #0 to host tapi.telstra.com left intact
NULL