We have recently responded to a customer request by adding an API which allows customer to update an sms contact using a simple web request. This means that customers can automate the changing of their primary on call number as part of their standard operations procedures.
Using a standard URL customers can now quickly update their mobile number without having to use the control panel. This could be acheived by a simple browser bookmark or using a command line program such as wget as part of a script.
Contact support for more details.