The random dial tone is most likely coming from Skype for Business
How to get ride of the random dial tone while on PC Mode:
1) Right click on the speaker located in the bottom right hand corner of the computer
2) Hit SOUNDS
3) Go under the SOUNDS tab
3) Under PROGRAM EVENTS search for “Skype for Business” or “Microsoft Lync”
4) Then select DIALTONE and under the “Sounds” section set it to NONE
5) Go back to DIALTONE and select HOWLER SOUND this time
6) Under the “sounds” section set it to NONE
Try this if you see “Open Sounds Settings” instead of “Sounds” proceed below.
Once we click on “Open Sounds Settings” , look the the right hand side of the window under “Related Settings” and select “Sound Control Panel” NOW go back to step 3 above.