Incoming calls whilst on a call

[Hardes]

My phone (iPhone) is not showing me any incoming calls when I am on a call, and it goes directly to voicemail and I receive a text message telling me the number has tried to call me.

How do I activate incoming calls to be shown, giving me the usual options of answer and put on hold, or answer and hang up the other call?

I’m rather new to O2 and this never happened with my previous operator. I have the same handset and the only thing that has changed was changing to O2. 

thanks in advance 

[Cleoriff]

@Hardes 

You  could try O2 on 202 or 0344 809 0202 but I doubt they would be much help.

Apple would be your best option for an answer to your query: Official Apple Support

[MI5]

@Hardes 

Settings >Phone >Call Waiting and turn on.