Hi ICDIT, I think there might be a few issues - you may already have considered them, and have solutions available, but here are some of my thoughts.
If your planned system is in place, whoever has the phone with the app, has access. Someone has a phone with the security credentials, and they can get airside, without dealing with airport security or showing their ASIC to the ARO or anything like that - while that might be the aim of your app, it has downsides too, you have to have some method of preventing other unauthorised users from getting the app and installing it, possibly with faked credentials. Issuing and revoking these credentials will require some infrastructure. This is before you get into the realm of attempts to break the coding in the app, or spoof the communications method.
If you are counting on this system to provide real-time access control or denial, checking against a central server that the carrier is authorised to enter at the time of entry will only work where the phone is charged and has network coverage (either on the phone, or the gate controller, depending on the design) to confirm that access is allowed. There are plenty of places out there that dont have network coverage, or only have Telstra, which could be a hurdle. How are you planning on using the app? Tap the phone against a reader? Enter a user specific PIN into the phone? Send a text to a designated number? The text method relies on network coverage. In other cases that dont directly use the mobile telephone network, what communications technology will you use, and is it fitted to a significant majority of phones? If your plan involves getting a large number of people to upgrade their phones it wont be practical, and any solution has to work with both Apple and Android/etc phones.
Thinking about the more remote airports that still have security gates, but typically have noone present most days of the week, a high-tech powered gate at remote airports may be vulnerable to weather or vandalism, where a mechanical push-button lock isn't, and will require power, maintenance and most likely some form of network connection to check whether the phone requesting access is allowed to enter the airport. This network connection could be the most problematic, hypothetically if it breaks, at remote ports this most likely wont be discovered until the next pilot tries to get back to their aircraft, and it may be a week or two until the repairs can be made after Farmer Joe inadvertently cut a cable or the radio transmitter is hit by lightning, so there still needs to be a backup method such as the currently existing push-button locks. If the backup method still works and has to be kept, it'd be hard to argue to add an extra layer of technology and expense at so many remote airports. There's a few of the things I'd consider in the planning of your idea
