SIP-Account: End-to-end encryption possible?

You can use different encryptions for SIP-Accounts but will a SIP-call using Jami always be end-to-end encrypted?

Even if you use encryption protocols it won't be always end-to-end encrypted?

https://www.64characters.com/blog/whats-new-in-telephone-15/

For example Linphone only allows end-to-end encryption if you use Linphone and "sip.linphone.org" accounts:

https://www.linphone.org/news/end-end-encryption-1-1-and-group-instant-messaging

Is it possible to achive end-to-end encryption using SIP-accounts (any provider) and the Jami client?

• In that case every peer has to use the Jami client and a SIP-account?
• If another peer uses the Plain old telephone service (POTS) and/or the SIM-card of a cell phone to join a call - the call won't be end-to-end encrypted anymore?

https://www.metaswitch.com/knowledge-center/reference/what-is-session-initiation-protocol-sip