Not sure why your system does not work. This works fine for me

curl -XPOST https://platform.devtest.ringcentral.com/restapi/v1.0/account/[accountid]/extension/[extensionId]/sms -H 'accept: application/json' -H 'content-type: application/json' -H 'authorization: Bearer valid-access-token' --data '{"from":{"phoneNumber":"xxxxxxxxxx"},"to":[{"phoneNumber":"xxxxxxxxxx"}],"text":"test test and test again"}'

Can you try with "test%20test"

This is something wired ... I never faced this issue.. Just see I am getting the response perfectly

Make sure you using correct Json POST request as below format and structure:

{"from":{"phoneNumber":"your phone number"},"to":[{"phoneNumber":"Your number to send"},{"phoneNumber":"Your number to send"}],"text":"Test message with space"} 

Please check the phone numbers as well, it needs to be in E.164 format - Also, what is the error message you are facing? Can you elaborate?

FileMaker encodes the data in a weird way...To get spaces in the text body to produce valid code, the --data must be used from a variable:
Set Variable $data ; "{"from":{"phoneNumber":"" & $from & ""},"text":"" & $message & "","to":[{"phoneNumber":"" & $to & ""}]}"

then the cURL must reference "-d @$data" as in:

"--request POST" & ¶ &
"--header "Accept: application/json"" & ¶ &
"--header "authorization: Bearer " & $token & """ & ¶ &
"--header "content-type: application/json"" & ¶ &
"-d @$data"