When your are in the office you can control your desk phone from the App on your Computer
Click the phone button and choose Deskphone(CTI)
This now let you dial form the app but talk though the phone
This is very useful when copying and pasting numbers from emails / spreadsheets / websites etc
To
Please remember to turn this off when you are out of the office otherwise when you press answer on your computer the call will be happening on your desk phone