If the host is an Internet telephony gateway, the user field MAY also encode a telephone number using the notation of telephone-subscriber. The telephone number is a special case of a user name and cannot be distinguished by a BNF. Thus, a URL parameter, user, is added to distinguish telephone numbers from user names. The phone identifier is to be used when connecting to a telephony gateway. Even without this parameter, recipients of SIP URLs MAY interpret the pre-@ part as a phone number if local restrictions on the name space for user name allow it.

The SIP scheme may use the format "user:password" in the userinfo field. The use of passwords in the userinfo is not that intelligent, because the passing of authentication information in clear text (such as URIs) has proven to be a security risk in almost every case where it has been used.

The mailto: URL and RFC 822 email addresses require that numeric host addresses ("host numbers") are enclosed in square brackets (presumably, since host names might be numeric), while host numbers without brackets are used for all other URLs. The SIP URL requires the latter form, without brackets.

The port number to send a request to. If not present, the procedures outlined in Section 1.4.2 are used to determine the port number to send a request to.

SIP URLs can define specific parameters of the request. URL parameters are added after the host component and are separated by semi-colons. The transport parameter determines the the transport mechanism (UDP or TCP). UDP is to be assumed when no explicit transport parameter is included. The maddr parameter provides the server address to be contacted for this user, overriding the address supplied in the host field.  This address is typically a multicast address, but could also be the address of a backup server. The ttl parameter determines the time-to-live value of the UDP multicast packet and must only be used if maddr is a multicast address and the transport protocol is UDP.

Headers of the SIP request can be defined with the "?" mechanism within a SIP URL. The special hname "body" indicates that the associated hvalue is the message-body of the SIP INVITE request. Headers must not be used in the From and To header fields and the Request-URI; they are ignored if present. hname and hvalue are encodings of a SIP header name and value, respectively. All URL reserved characters in the header names and values must be escaped.

The method of the SIP request can be specified with the method parameter.  This parameter must not be used in the From and To header fields and the Request-URI; they are ignored if present.