No engaged tone when I call someone

[BGee]

I am a new O2 customer from the Virgin transfer. The transfer went smoothly without me needing a new SIM card.

All seems to be OK apart from this one issue, when I call someone who line is engaged, I do not hear the engaged tone, the call is abandoned and I see the message 'call ended'. It is disconcerting because it could be that the person is engaged, or it could be that something is wrong with the line. How do I get the engaged tone?

[MI5]

@BGee 

Depends on how the person you are calling has set their phone up.

They may not have call waiting enabled.