I bought a Panasonic cordless phone system, disabled the included answering machine and now calls will not go to OOMA's voicemail. They did before on my old phone, without the built in answering machine.
Goto https://my.ooma.com/voicemail and see how many rings you have set for "voicemail pick-up time"
Is it 24 seconds or 4 rings? You can customize this but make sure you don't have it set to 59 seconds or something like that....
What happens if you call your own number and it rings 4 or 5 times?