Where is the voicemail system located, is it part of the phone or a separate unit, or a hosted service, e.g. with Spark One etc.  If hosted then (and it's maybe) sometimes the ring time before divert to V/M can be extended, either via a code you dial on the phone, or the service provider has to do it.  If part of the Doro phone. or a stand-alone, (maybe) the V/M unit has a setting (either via a switch, or a code you dial) to extend the ring time.  I suspect either way, Google will be your friend here, if you Google the make/model and see what you get.