ringing mobile when engaged

[BETHIE20]

I am with vodafone for my mobile.  So is a visitor to my 4 storey househouse.  I have an iphone, they have an android.  Sometimes when I phone them while they are in the house (I cant face the stairs!)  their  phone seems to ring even tho they are engaged...they get a call waiting sign.  This can go on so long I go to check on them to find they are on the phone,....hence discovering  that while their phone seems to ring  they are engaged...and they  get a call waiting sign up.  

Why is this?

[AnnS]

The person you are calling has call waiting set up in the call settings @BETHIE20 , they would be aware you are waiting and would receive a beep and your phone would continue to receive the tone until they ended the call they are already on.

[BETHIE20]

Thank you for your prompt response.  

But why do I hear the ringing tone...not an engaged one?  Are you saying that when someone has call waiting set up the caller gets a ringing tone if the person called has call waiting set up  and is engaged?

[AnnS]

The line is not engaged @BETHIE20 , the call is waiting to be answered and this is someting the receiver is well aware off, they would constantly hear a beep until they ended the call they were on or sent your call to voicemail.

The only way to get an engaged tone is to also turn off voicemail, otherwise you will receive a ringing tone even if the receiver is on a call.