Help Ringtone problems

[daveo72]

I am having problems when certain people call me. I haven't set anything up but my phone will only ring once (sometimes not at all) when certain people ring. I get a speaker icon with a line through it along the top of my phone then my ringtone stops. How can I change this so it carries on ringing. I've checked my settings and all volumes or on full. Like I say I haven't set anything up on my phone it has just happened. Can anyone help please.