US monitored the phone calls of 35 world leaders: Report

Voip phones over Internet will be the easiest to be monitored & recorded & replayed - because they are converted from analogue to digital - which can be stored in harddisk, and playback - and run over Internet.

Analogue phones are going over private PABX and PSTN - that is controlled by private Telcos. Harder to hack - unless tap the phone line.

Voip go over Internet are basically data (which can be siphoned off) - even encrypted - can be hacked if it compromise the encryption key, encryption alogrithm.

Video conferencing over IP via Internet - lagi better. The whole video conferencing session - with image and voice and whiteboarding can be recorded and playback - unless using dedicated ISDN lines - point to point with encryption - will be safer.
 

Mobile phones (smartphone) has accessed to 2 infrastructure - voice and data (via Internet) - and can be hacked coming from Internet.

(After note :- VoIP use call manager via SIP or H.323 to do call admission, address search and do call setup. Call manager is also a server that can store voice call as digital data, do recording, do tracking, do logging and can playback - and can even pipe to external server for external storage.

Similarly, video conferencing also used H.323 to do video call setup, make reservation for meeting, administer video session with multiple parties. Video conferencing is also a server that can record the whole conferening session, allow whiteboarding for collaboration, do recording, do tracking, do logging and can playback - and can even pipe to external server for external storage.)

e.g. of logging configuration to external server :-

logging on
logging host 12.1.1.1
logging console criitical
logging trap informational