Why doesn't my phone always ring when someone calls me

Sometimes, whilst your phone is in your pocket, you can accidentally turn on silent mode.

However, this is unlikely. The most likely reason is because at the time of them calling you, your device may not have service, therefore the call will not be received yet a 'missed call' notification will appear when it recieves service again.

There shouldn't be any particular issue with your phone.

It's because in the tools under do not disturb you get two choices under silence you can either silence "always" or "Only while iPhone is locked". So if you can find a fix for that I would be interested as well. T Mobile workers don't know how to get my phone to ring even though they sold it to me.