* Voipfone Softphone (Android) deserves a mention. It needs an account with voipfone.co.uk, but you can signup and try for free. Their softphone app just works out of the box for calling SIP URIs (although an update in March 2019 has stopped this working)
Some clients can call SIP URIs without any configuration, other clients require that a SIP account is defined (and perhaps enabled). For these 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.
==Known not-working clients==