Hi:
We'd need some additional details to fully understand what's going on.
Which service provider are you using -- Google Voice, or a SIP provider like Phonepower, Anveo, Callcentric,
voip.ms?
If it's Google Voice, keep in mind that GV has its own VM system, which cannot be disabled, and you may be running into contention with GV trying to handle the call.
What do you mean, exactly, by "ring tone"? Do you mean the sound you'd hear when placing a call and hearing ringback tone while waiting for the other party to answer? Listen to the sample file attached. Is it that, or some other sound?
Here is a complete SWAG:
Log into the OBiTALK portal, click on your OBi 100 device, and get into the Expert mode menu. Go to Physical Interfaces --> Phone. Scroll down to find the Timers section. Uncheck the OBiTALK Settings box and the Device Default box next to CPCDelayTime, and change it from the default 2000 milliseconds to 1000. Click the button at the bottom of the page to submit the change, and give the OBi a minute to reboot. See if that changes the behavior on the answering machine. If it still makes the sound, but it's shorter, then try changing the value to 500, submit, test again, and see what happens.