You may want to automatically stop call attempts given a lead status change in Salesforce.
To do this, you just send an Outbound Message from your workflow rules to unpark or stop the lead calls.
Configure your Outbound Message as follows:
Name = Speak2Leads Stop Calls
Unique Name = Speak2Leads Stop Calls
Endpoint URL = https://s2l.speak2leads.com/main/salesforce/unpark
Include the following fields in your Salesforce Outbound Message to Speak2Leads