You didn't mention which telephone service provider you are using.
Typically, internet telephone service providers will drop calls after two or three hours, as a way to ensure that their resources aren't being tied up by a call that just got left off-hook or failed to be terminated. This also protects their users from accidentally being billed for long distance charges on calls that failed to end. Just dial back in again. There is no setting on your OBi device to control this, although some service providers have an account setting to adjust the maximum allowed call time.