To send a new request, you should use one of the functions in nksip_uac module. NkSIP supports all defined SIP methods: OPTIONS, REGISTER, INVITE, ACK, BYE, CANCEL, INFO, UPDATE, SUBSCRIBE, NOTIFY, REFER, PUBLISH and MESSAGE (PRACKs are sent automatically by the nksip_100rel plugin).
Depending on the specific method, the request should be sent out of any existing dialog or in-dialog. Out-of-dialog sending request functions will need a Service name (user name or internal name), a sip uri to send the request to and an optional list of options. In-dialog sending request functions will usually need the dialog's id or subscription's id of the dialog or subscription. You can get them from the returned meta values of dialog-generating functions (like INVITE or SUBSCRIBE) or from sip_dialog_update/3 callback function.
The returned value can include some metadata about the response. Use the option meta to select which metadatas you want to receive (see metadata fields). Some methods (like INVITE and SUBSCRIBE) will allways include some metadata (see bellow). You can use the Dialog API functions to get additional information.
You can define a callback function using option callback, and it will be called for every received provisional response as {reply, Code, Response, Call}. Use the functions in Responses API to extract relevant information from each specific response. It's important to notice this callback function will be called in the same process as the call (see explanation in callback functions), so you shouldn't spend a lot of time inside it. You shouldn't also copy the Response or Call object to other processes, because they are quite heavy.
By default, most functions will block until a final response is received or a an error is produced before sending the request, returning {ok, Code, Meta} or {error, Error}. You can also call most of these functions asynchronously using async option, and the call will return immediately with {async, RequestId} before even trying to send the request, instead of blocking. You should use the callback function to receive provisional responses, final response and errors. RequestId is a handle to the current request and can be used to get additional information from it before it's destroyed (see, the Requests API) or to CANCEL the request.
Most functions in this list add specific behaviour for each specific method. For example, invite/3,2 will allways include a Contact header. You can use the generic request/3,2 function to avoid any specific addition.
In case of using a SIP URI as destination, is is possible to include custom headers, for example <sip:host;method=REGISTER?contact=*&expires=10>, but it must be escaped (using for example http_uri:encode/1). You should use request/3,2 if you specify the method in the uri.
See the fill list of sending functions in Sending Functions, and the full list of options in Sending Options.