In order to troubleshoot this, you'd need to start by trying to isolate the issue to either a Google XMPP (Chat) issue, or an OBi issue, or an issue with your home network. As is, it's impossible to determine the root cause.
You can start by removing your OBi from the environment. Unplug it from power. Use the Firefox web browser on a PC or Mac, and download and install the Google voice & video plugin (AKA the Hangouts plugin). You'll need to tell Firefox to allow, or activate, the two plugin components, since Firefox blocks them by default.
Log into Gmail (not GV), making sure you are using the same Google account as you are using with your OBi. Click your chat avatar on the left side of the Gmail web page, to reveal the settings box. Look at the bottom of the box. If it says "revert to old chat", click that. If it says "try the new Hangouts", don't click it.
Now, make some calls via Gmail, and find out if the calls drop at 14 minutes. If so, post your issue over on the Google Voice help forum. If not, then you've got an OBi issue.