ing specified in the URI. If the parameter user=phone is present, then the username portion of the URI can be interpreted as a telephone number. This allows additional parameters in the username portion of the URI. This allows the parameters and structure of a tel URL [19] to be present in the user part of the SIP URI as described in the next section. The sips URI scheme has the same structure as the sip URI but begins with the sips scheme name. Note that a sips URI is not equivalent to a sip URI with transport=tls, since the sip URI does not have the same secu- rity requirements as the sips URI. The requirement is that TLS transport is used end-to-end for the SIP path. The only exception is hop between the final proxy and the UAS, which may use another security mechanism besides TLS (IPSec, for example). Not shown in the example is the loose route parameter lr, which can be present in sip or sips Record-Route and Route URIs to indicate that the proxy server identified by the URI supports loose routing. 4.2.2 Telephone URLs The telephone URI scheme, tel, [19] can be used to represent a resource iden- tified by a telephone number. Telephone numbers can be of two general forms, local or global. A local number is only valid in a particular geographic area and has only local significance. If the number is used outside of this area, it will either fail or return the wrong resource. A global telephone number, also called an E.164 number, is one that is, in principle, valid anywhere. It contains enough information about the country, region, and locality SO that the PSTN network is capable of routing calls to the correct resource. An example of a local phone number is: tel:411. ;phone-context=+1314 which indicates a call to directory assistance valid only within country code 1 and area code 314 as identified in the required phone-context parameter. An example of a global phone number is: +13145551212 Global phone numbers always begin with the "+" identifier followed by the country code, 1 in this case, followed by the re