Curl perform failed: Couldn't resolve host name (6)

There is exactly one other post on the forum with this same issue, and it was related to OTP.

https://devcentral.f5.com/questions/otp-generation-on-f5-and-sending-to-clickatel-for-two-factor-authentication

So the error suggests that the OTP provider here can't resolve the hostname, which implies a DNS issue. Do you have DNS configured on the box? Can you resolve the hostname from the command line?