Get remote signing URL in Salesforce

I have a requirement to trigger sending of a document via DocuSign for remote signing. The signing link for the documents needs to be sent as a text message (SMS) using a custom solution (Twilio). Using Apex Toolkit dfsle.EnvelopeService.sendEnvelope(myEnvelope, false), I’m able to generate an envelope with a Template. Now the challenge is to get the signing URL. I have been able to get the recipient preview URL using dfsle.SigningService.getRecipientPreviewUrl(docuSignUUID,1) but it can’t be used to complete the document. I have tried using dfsle.SigningService.getSigningUrl(envelope, returnURL ,true) but it alwasy returns a NULL value.

Is it possible to achieve what my requirement says? If yes, can someone throw light on what I’m missing or have been doing incorrectly?