why does my iphone only ring twice ?

As a user, you have no control on how many times the phone rings. That is done by the carrier. It actually is not based on the number of rings, but time. It is determined by the amount of time the call takes to setup and deliver to the handset. If it appears you are only hearing the phone ring twice, you can contact the carrier and see if they can adjust your account to wait longer before the call goes to voicemail. However, be warned that if they are able to adjust it, it will only be a short period. Some carriers cannot adjust the timing.

There is no setting on a phone to control how long it will ring. Some cellular carriers have an option for a longer ring time. You may want to contact your carrier customer service.