Maximum ring time is typically a voip account setting at your voip provider. If you have an answering machine or service that may also affect the ringing time.
Your phone should ring until sip signalling from your voip provider sends a signal to stop it or the adapter senses off hook on your handset as a call answer.
For the 30-minute disconnect that may also be a setting or policy of your voip provider. If your voip provider cannot help, it may be a result of some other failure. I would run a sip trace to see if that gives a clue to the cause of the disconnect.