Call Barring

Clearwater allows users to configure rules for blocking of incoming or outgoing calls, by implementing the IR.92 version of call barring (which is itself a cut-down version of the MMTEL call barring service).

The relevant specifications are:

Some of the entries in the IR.92 list are not currently applicable for Clearwater, since Clearwater does not have support for/the concept of roaming calls. Because of this, the customer may only specify that they wish to block one of:

  • No outgoing calls
  • All outgoing calls
  • Outgoing calls to international numbers (but see below)

And one of:

  • No incoming calls
  • All incoming calls

International Call detection

The 3GPP specification states that to detect an international number, a SIP router should discount SIP/SIPS URIs that do not have a user=phone parameter set on them (this parameter is intended to distinguish between dialed digits and call-by-name with numeric names) but X-Lite and other SIP clients never set this parameter. Because of this behavior, the Clearwater implementation considers all SIP URIs for international call categorization.

National dialing prefix

Another limitation is that the international call detection algorithm only checks against a global ‘national’ dialing prefix (which is fine unless subscribers are expected to be multi-national within one Clearwater deployment). There does not appear to be a mechanism in IMS networks to store a subscriber-specific locale (e.g. in the HSS) so this would require some smarts to solve. These issues may be addressed in subsequent development.