(Bit of a rewrite to tidy things up)
(Hmm, refer to numbers and the first examples are names...)
[[File:VOIP-sipURIcall.png|thumb|Calling a SIP URI with the 'Ekiga' softphone]]
With SIP, it's possible to make data calls over the Internet to SIP accounts by using URIs of the form ''sip:
* the acclaimed 'Lenny 'service is supposedly available at sip:email@example.com
Incoming calls can be made from the Internet to
Most SIP clients which support calling SIP URIs are softphones. When testing, some clients can call SIP URIs without any configuration, other clients can call SIP URIs but require that a SIP account is defined (and perhaps enabled) although they don't actually use the account to place the call. There are also other clients which ''appear'' to support calling SIP URIs but which strip off the domain and call the number part as a chargeable phone call.
For testing clients, it can be useful to have a free SIP account which can't place external calls (e.g. [https://www.linphone.org/freesip/home linphone]) - that way one can be sure that the client isn't making chargeable calls.