As has been indicated above, it's your phone service provider that determines the maximum ring time but it's not usually above 30 seconds. (A ringing phone occupies a 'line' on a mobile network. As there are only a limited number of 'lines' possible, mobile phone companies can't allow people to 'hold on' for ages while the phone that they're trying to contact is ringing for several minutes).
The maximum ring time is often 30 seconds but some phone services only allow 25 seconds. With 'virtual' phone service providers (such as GiffGaff, Asda Mobile, Tesco Mobile, etc), who don't have their own transmitters, you usually need to 'make a phone call' to the number used by the relevant host company (that actually owns the transmitters). So, for example, for example, GiffGaff customers should try the number for O2, whereas Asda Mobile customers should try the Vodafone number. (Note though that not all 'host' companies allow customers of the 'virtual' ones that use their transmitters to change the setting).
So try the relevant 'phone number' from Etch's link but replacing '20#' with '30#' at the end. You should see a message on your phone's screen to tell you whether or not it's worked. If it hasn't, try again with '25#' at the end of the number.