Hi,
OnionCat provides a public overlay network. If a SIP-proxy ist put on 
top of OnionCat any dumb hardware-/software SIP-phone in the LAN can be 
used.
On the OnionCat-interface I suggest to use
1. DTLS/SRTP with self-signed certificate created from the Tor Hidden 
Service hostname/private key for authentication and encryption
2. OPUS audio codec with forward error correction enabled as it is the 
most robust audio codec and adjusts bandwidth automagically
3. Use speed dialing capable proxy for hardware phones top map decimal 
numbers to SIP-URIs (blockchain as long-term goal, see 
https://forum.namecoin.info/viewtopic.php?f=2&t=1858)
I also suggest to consider RTCWeb as Tor pluggable transport for 
OnionCat as it is encrypted by default, can multiplex multiple streams 
and ordered mode/congestion control can be switched on/off per stream.
Best regards,
Renne