This is typically caused by either your Internet Service Provider (ISP) or some other, heavy use on your home network, such as online gaming, HD video streaming, etc.
Typical home internet service is asymmetric: the download speed to your end is much faster than the upload speed. When you are talking, you are using the less-capable upload link, which can be more susceptible to degradation.
Cable internet service is also susceptible to problems with the coaxial cable delivering the signal to your modem. Rainy/snowy/icy weather can cause water incursion into the cables and their connectors, cables can get damaged by construction, rodents or tree roots, and connectors can get corroded.
Basic speedtests are meaningless for VoIP. You can, however, look at your call quality in real time. To do this, you can log into the OBi's local web server. Pick up the phone attached the OBi and key in ***1 to hear its IP address. Go to that IP address in your browser. The default login is admin/admin. During a call while you are talking, go to the Status-->Call status page. You should see statistics about the call, including a MOS number. A MOS lower than 4.0 indicates a problem with your internet service.