Why does my iPhone app's web service call fail on AT&T 4g LTE only?

Question

I have an iPhone application that makes a few web service calls. There are 2 calls to simple RSS feeds and then there are 2 calls made to an ASP.NET powered web service.

The calls to the RSS feed work consistently in every scenario whether it is via wifi or 4g, and it also works for every carrier (AT&T, Sprint, Verizon, etc...).

The 2 calls made to the ASP.NET web service will always work on wifi, but they will not work on 4g with every carrier. Specifically AT&T 4g LTE will fail on these calls immediately every single time whereas calls made by another carrier (CSpire) will succeed on 4g.

I want to clarify that iPhones using the same version of the app will succeed when requesting the ASP.NET calls via wifi, but will fail only when using AT&T 4g LTE. I have tested them myself with "full bars" (full network service access) immediately after visiting websites on 4g.

I do not have access to any error messages as I do not personally own an iPhone with AT&T as its carrier.

I've looked online and haven't found any issues related to a specific carrier like this. Has anyone ever heard of this before?

EDIT: I have not been able to test the iPhone app on a device with T-Mobile as the carrier, but I have learned that AT&T and T-Mobile use GSM while the other carriers use CDMA. Could this be a determining factor?

Answer 1

I ran into a very similar issue with a project I'm working on. The project uses authenticated soap web services that are implemented in ASP.net.

The problem ended up being that AT&T has an http proxy. I was accessing the authentication web service through http so it was being proxied. The call to subsequent services was being done over https (which can't be proxied) so the authentication token I received over http was not valid. Switching all the service calls to https (including authentication) fixed the issue for me.