As Apple releases more to the service providers they will be in stock. Estimated time frame is now till next year.
You can also check the status daily here: iPhone
Also, Please not that this out of stock issue is not the fault of any cellular company and is purely how the stock is kept in on Apples side and shipping issues.
One of the big reasons is due to Samsung's meltdown with their new phone released prior to the 7 coming out. This caused an even heavier demand for iPhones than normal.