https://jmp.chat/ is a good alternative. Text and picture messaging over XMPP, and voice over real SIP (with voicemail transcribed to text).

There are lots of great XMPP clients, such as Conversations on Android, and Gajim or Adium on desktop. Or use https://movim.eu/ if you like a web client. iOS has a few options as well, including Tigase Messenger and IM+.