As we can see in this picture, there are the following telephone sets on the president's desk in the Oval Office:
- On the left: a new Cisco 8841 IP phone, which is part of the internal White House telephone network and can be used for all non-secure calls. This network was modernized since 2015, but the phone was not placed on the president's desk until late 2016.
On the back of this phone is a black metal box and on the front panel there's an extra red button, both of which are modifications by Advanced Programs, Inc. (API) in order to meet Telephone Security Group (TSG) standards, including on-hook security for the handset and the speakerphone and probably also for TEMPEST protection.
These modifications are to make sure that the phone cannot by any means be caused to produce or transmit audio when the handset is on-hook - whether accidental or deliberate. It's not for securing the calls by means of encryption.
- On the right: an older Cisco 7975G Unified IP Phone with expansion module 7916, which is for the highly secure Executive Voice over Secure IP-network connecting the president to all major decision makers, like the secretaries of State, Defense and Homeland Security, the Director of National Intelligence and top military officials.
This telephone network became operational in 2006 or 2007, but this particular phone is on the president's desk only since early 2011. Elsewhere, these phones usually have a bright yellow bezel or faceplate instead of the standard silver one, indicating they may be used for conversations up to the level of Top Secret/SCI.
http://electrospaces.blogspot.com/2017/02/trumps-beautiful-oval-office-phones-and.html