Ok, so CSipSimple, Lumicall etc are pretty much dead on Android 14. Linphone and SipDroid are alive, bu the second one is unusable. So, which FOSS alternatives of Linphone exist at this point?

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SIP providers are plenty: Sip2Sip, OnSIP, AntiSIP, pbxes, Ekiga, IPtel, Linphone itself, not to mention various SIP-to-PSTN providers like ippi and various Dellmont clones. For pure SIP, I used AntiSIP most.
Generally speaking, no. But I don't remember full implementation details, maybe there are some options to encrypt RTC data. I only remember that signaling is encrypted separately.