What are some alternatives?
When comparing Signal and Eir SIP Voice, you can also consider the following products
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Riot.im - Open source interoperable collaboration app.
VoIP.ms - VoIP.ms is a VoIP origination and termination tool.