How to make external calls:
https://www.3cx.com/blog/voip-howto/make-calls-yealink-t42-t41/
https://www.3cx.com/blog/voip-howto/make-calls-yealink-t48-t46/

Internal calls:


Press 3CX PBook, navigate to the phone you would like to call, then press the soft button which corresponds to the phone name.

How to transfer calls:
https://www.3cx.com/blog/voip-howto/blind-attended-transfer-yealink-t42-t41/
https://www.3cx.com/blog/voip-howto/blind-attended-transfer-yealink-t48-t46/

How to check voicemail:
https://www.3cx.com/blog/voip-howto/voicemail-yealink-t42-t41/
https://www.3cx.com/blog/voip-howto/voicemail-yealink-t48-t46/