Difficulty making and receiving calls from some numbers

[Pigmy]

I'm having difficulty making and receiving phone calls from certain numbers, but can happily call others.  I can't call my partner's mobile (also O2) - it tries to connect then says "Call Ended" after a few seconds.  Same happened today to my Dr's surgery (local rate landline starting 02392).  But I can call my folk's landline and my work mobile (Vodafone).  My partner has had problems calling me but they are sporadic.  My brother's mobile couldn't call me last week as well.  Not sure what to do next.  None of the numbers are blocked (I've tried to block and unblock them) and they are typed in correctly.  Any clues? TIA

[MI5]

@Pigmy 

Start by checking for any network issues Guide: How can I sort out my network issues? 

If nothing showing, try a new sim.

You can pop in store, with photo ID, to get one for free.