I assume that your calls are going to your phone provider's voicemail service. If so, it's that phone provider that determines the number of rings, not your phone.
e.g. if (say) Vodafone sends a call to your phone, Vodafone's computer will monitor how many times your phone has rung and then, when it reaches a pre-set number, stop sending the call to your phone and send it to Vodafone's voicemail system instead. (The make a model of your phone is therefore totally irrelevant).
So, if we're to try to find out how you can change the relevant setting, we need to know which service operator (Vodafone, EE, etc) you're with.