I got the same message today when trying to deactivate the EE/BT network answerphone service for my Mum as she is getting in a right muddle having always used her answerphone at home. For some reason BT were able to offer the ability to deactivate the service, but EE are not (despite the two companies being the same now).
Unfortunately setting the ring length to the maximum only works as a solution if you have your voicemail switched on at home and the ring length for your home phone kicks in sooner than the network based answering machine. If you have not switched your answering machine on because you are home, but unavailable at that time to take the call instead of ring, no answer, the network service still kicks in allowing messages to be left there.
The only option I was left with was to leave a greeting on EE/BT's network based answer phone service asking people not to leave a voicemail here and to try again later due to a limitation with BT/EE's network based answer phone service. I also use the time wisely why I have their attention and suggest to people to not consider moving to EE/BT for their landline services as in my experience you'd get more sense talking to a brick wall.
Looking forwards to the time my contract expires and I can leave EE/BT.
