Hi:
It is very likely your WiFi signal, unless you happen to have a problem with your Internet service. Raw internet speed is not the primary factor for reliable VoIP. When you watch a video or download files, the systems can buffer the data to make up for dropouts or other signal problems. VoIP is real-time, and it's highly sensitive to network problems.
Plug the phone into an Ethernet connection, and see if the problem goes away. If you still have the problem, then you have an ISP or LAN issue. If the calls are very clear in both directions over Ethernet, then you have a WiFi issue.
The range on the OBiWiFi connections isn't that great. If you have a dual-band router, try connecting via the 5GHz channels and via the 2.4GHz channels and see which one works best for you.