Windows Mobile 6.5 SIP Settings for VoIP

Windows Mobile 6.5 included an integrated Voice over IP (VoIP) client powered by a native Session Initiation Protocol (SIP) stack embedded directly into the operating system. Although many mobile carriers suppressed the user-facing interface, the underlying operating system supported full internet calling capabilities through the native dialer. This article outlines the built-in SIP configuration options, provisioning parameters, and call routing settings available in Windows Mobile 6.5.

Account Credentials and Authentication

To authenticate with an IP PBX or VoIP service provider, the Windows Mobile 6.5 SIP client required fundamental authentication parameters, typically configured via XML provisioning files (_setup.xml) or direct registry entries under HKLM\Comm\RTC\SIP:

  • Display Name: The caller ID name presented to call recipients.
  • User Name / SIP ID: The user extension, telephone number, or account identifier.
  • SIP Domain / Realm: The host domain of the SIP service (e.g., sip.provider.com).
  • Authentication User: The specific authorization username required by providers separating account numbers from auth IDs.
  • Password: The alphanumeric secret key used for Digest Authentication.

Server and Proxy Configurations

The native stack supported standard SIP server definitions to manage registration and routing:

  • Registrar Server: The Fully Qualified Domain Name (FQDN) or IP address of the SIP registrar, alongside the listening port (defaulting to UDP 5060).
  • Registration Interval: The expiry time (in seconds) between registration refresh requests (typically set between 120 and 3600 seconds).
  • Outbound Proxy: An optional proxy IP address and port used to route SIP traffic out of private local networks, aiding in NAT traversal.

Transport and Network Binding

Windows Mobile 6.5 allowed administrators to define how and when SIP traffic moved across the device’s data interfaces:

  • Transport Protocol: Selection between UDP (User Datagram Protocol) and TCP (Transmission Control Protocol) for signaling transmission.
  • Network Interface Priority: Rules governing whether SIP services should run exclusively over Wi-Fi connections, cellular data (GPRS/EDGE/3G), or switch automatically between them.
  • Keep-Alive Timers: Configurable packet intervals designed to maintain open NAT pinholes for incoming call signaling without draining the device battery.

Internet Calling Rules and Dialer Integration

Once configured, the Windows Mobile 6.5 dialer integrated internet calling alongside standard cellular telephony. Users could configure call-placement rules:

  • Always Call via Internet: Automatically routed all dialed numbers through the configured SIP account whenever a viable data connection was available.
  • Only When Cellular Is Unavailable: Used cellular networks as the default, only switching to internet calling in dead zones with active Wi-Fi.
  • Ask Each Time: Prompted the user to choose between "Cellular Call" or "Internet Call" whenever placing an outbound call.

Audio Codecs and Media Handling

The built-in Real-Time Transport Protocol (RTP) engine supported several industry-standard audio compression algorithms. Priority levels and payload formats could be configured for codecs including:

  • G.711 (u-Law and a-Law): Standard uncompressed VoIP codecs prioritized for high-bandwidth Wi-Fi networks.
  • GSM 6.10: Low-bandwidth codec optimized for cellular network environments.
  • AMR (Adaptive Multi-Rate): Mobile speech codec utilized across supported 3G implementations.