When you have trouble, is your outbound audio completely silent, or is it choppy, distorted, noisy, etc?
What have you done to troubleshoot the problem? For example, if you are using the OBi with Google Voice, have you confirmed that you don't have the issue when calling from Gmail? Do you have the problem with a different provider? Does immediately calling back usually result in an ok call?
What kind of modem do you have? Separate router, if any? Contracted upstream speed? Upstream speed measured by speed test?