My phone doesn't ring... SOMETIMES

Okay so I've had an issue with my iPhone consistently pretty much since I've owned it though it's really starting to bother me.

The issue;

My phone will ring with contacts I have in my phone book though it seems any landline call I get (being any 03 9000 0000 or 03 8000 0000 number) or private call goes straight to voice mail. I constantly see 'missed call' and an annoying 'call catcher' SMS.

Do Not Disturb is not turned on. I have no idea what other reason this could be happening.

Any advice or assistance would be greatly appreciated.