I keep missing calls when the phone is in the other room, because the handset is set to increasing ring, and it’s really only getting going when the No Reply divert kicks in. Today I got up off my arse and looked into this, and discovered that you can extend – or shorten of course – the divert time using a GSM network code, as follows:
VOICEMAIL is the actual number of your voice mailbox, which you can read about here if you don’t know already; 10 sets it for all types; and of course SECONDS is the number of seconds you’d like it to ring for. You can choose anywhere between 5 and 30 seconds, personally I’m finding 20 seconds about right.
To check it’s run correctly, dial *#61#.