maining telephone digits. A tel URL can also contain some characters and information about dial- ing strings and patterns. For example: SIP Request Messages tel:#70555-1212;isub=1000 In this example, the dialed digit string, interpreted by a PSTN gateway, would be the DTMF digit # then 70 (to cancel call waiting, for example), then the digits 555-1212. Additional parameters include an ISDN subaddress of 1000. This example shows both types of optional visual separators allowed, either "_" or "." " " as the separator. Tel URLs can also be embedded in Web pages and could be included in HTML as, for example: Click <A REF="tel:+1972.555.1212">here</A> to get information about Dallas. The syntax and parameters of the tel URL may be used in the user por- tion of a sip URI. For example, the first tel example could be represented as a sip URI as follows: sip: :411%3Bphone-context:3D+1314@gateway.example.com The SIP URI adds a domain portion which represents the domain or gate- way that will route the request. Note that some of the characters in the tel URI must be escaped if they are included in the user part of a sip URI, in this example the ";" and "-" are escaped. 4.2.3 Presence and Instant Messaging URLs The pres URL scheme is defined [14] as a URL scheme that represents a "pre- sentity" or presence agent. The im URL scheme is defined [14] as a URL scheme that represents an "instant inbox" or an instant message client. Both URL schemes do not represent a new protocol but are resolved using DNS SRV resource records, which return another URI that indicates the actual presence or instant messaging protocol. For example, if the presence agent reference by the presence URL: pres:user@example.com supports SIP presence, the DNS SRV query would return a SIP URI, for example: sip:user@example.com which would then allow a presence agent to send a SUBSCRIBE to this SIP URI to obtain the presence agent of this user. The same procedure would be used for resolving an im URL into a SIP URI for sending a MESSAGE request. SIP: Underst