I don’t think you can change it, however if you wish to cancel the service just call 150 and speak to customer services they may be able to extend the number of rings but I’m pretty sure you can’t. You could use your own answering machine which can be set by yourself.
Have a look at Argos and Robert Dyas and some Ryman stores for landline phones with built in answering machines.
Once your voicemail service is cancelled don’t call 1571 again or this will automatically set up voicemail on your line again.
Your 1571 voicemail rings can be extended up to 10 rings 🙂
I have popped you a private message to take some information from you if you wish to go ahead with this change or you can call us on 150/03454541111 and our Customer Care team can arrange this for you.