I've replicated your solution and I got past the issue, the problem seems in the naming convention somewhere or in the SOAP call not including the correct headers SOAPaction headers, for testing I've named my assets as following
Schema: cus:smsApiService --> uppercase S for Service
JS: cus:smsApiService.js
Method: SendSingleSms
Payload
POST /nl/jsp/soaprouter.jsp HTTP/1.1
Host: localhost
Content-Type: text/xml; charset=utf-8
SOAPAction: cus:smsApiService#SendSingleSms
Content-Length: 303
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Body>
<SendSingleSms xmlns="urn:cus:SendSingleSms">
<sessiontoken>david/12345</sessiontoken>
<userid>12345</userid>
</SendSingleSms>
</soap:Body>
</soap:Envelope>

Make sure your are naming your function correctly

And firstly test it with just a simple string from your payload rather than a full request

SOAP request should be as follows

And it should include the SOAPAction header with value cus:smsApiService#SendSingleSms
Also check uppercase/lowercase in your schema/js code/function names, make sure it has the correct case.
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