After making the change, so far, it seems that the problem is no longer occurring. However, this is a temporary workaround, as I would like to have the message waiting indicator function. My phones do indeed have an LED that blinks when a message is available on the service provider. Like tismon, I also feel like the problem started after the latest firmware upgrade, although I really am not sure about that, it is possible it started before then. But in the last week the problem had become very frequent, sometimes happening once per minute, so that I had to just disconnect the obi.
Do you think this is something that can be solved by a firmware/settings fix (while keeping MWI function), or do you think there is a problem with our OBi's causing it, and need to replace the OBi's? Or maybe, the obi has some incompatibility with some phone models?
Note: for anyone trying this, I was using the settings interface that you get at the direct IP address of the obi (NOT via the expert config area of obitalk dashboard). The field is under Voice Settings-->SP1 (you'll have to do SP2 also if you have 2 service providers set up). For the X_VMWIEnable field, there were 2 checkbox options. I unchecked the one in the "Value" column, but after applying this and rebooting the obi, the setting would not stick (it became re-checked) and the problem persisted. So, I unchecked again, and also checked the box in the second column ("Default"), and I also did the same for the field above it "MWIEnable". After that, the settings seem to have stuck.