Hi Rajendra,
the sample pages for the SIPml5 JS library can be used to dial a fully qualified SIP URI. The interface will use the realm specified in the SIP stack config for short name/id strings entered. If you enter a full URI it will use it.
Because of media codecs and encryption you will be probably limited to G.711 voice calls unless you use a gateway like webrtc2sip. If the SIP points to an endpoint that will go via PSTN, then you will also need a subscription to a paid SIP service. This can also be configured into the stack so calls are routed through the correct service.
Warren