A list of SIP/VoIP services offering at no charge:
* a free account and SIP address
* SIP registration and SIP URI calling (always free)
* TCP transport for SIP signalling (reliable mobile VoIP, TLS)
* TLS for connection security and SRTP media encryption (to server)
* pass-through media or support for 'HD voice' wideband audio (OPUS, G.722)
* pass-through media or support for ZRTP end-to-end call encryption
A SIP client or device freely chosen by the user would be required at each end.
For added cost a service may offer DID origination and/or PSTN termination.
OnSIP - US servers
https://www.onsip.com/signup
SIP2SIP.info - EU servers
https://mdns.sipthor.net/register_sip_account.phtml
Telnyx - US (primarily)
https://sip.telnyx.com
IPTel.org - EU
https://www.iptel.org/services
Linphone - EU
https://www.linphone.org/freesip/home
OnSIP and Telnyx are enterprise-grade service providers.
None should be expected to supply basic SIP know-how.
All but IPTel and Linphone have FAQs, support articles.
Feel free to add to this list, elaborate on usage/features, report experience.
Ideally, in-service tenure of at least several years.
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