When Mobile Carriers Block Your App: Solving Hidden Content Filter Issues
Background and Problem Statement
A UK–based user reported that a mobile application for one of our clients is not working when the user was on mobile data. The application worked perfectly over Wi‑Fi and on other networks. The problem was confined to one SIM card, making it difficult to reproduce since our development team was based in India and the affected SIM was tied to a UK carrier. Initial troubleshooting suggested a network connectivity issue, but further investigation showed that other SIMs on the same phone, network settings and application builds worked without problems. This raised a suspicion that the issue was specific to the mobile carrier rather than the app or server.
Our Investigative Steps
Eliminating device and application settings
Our team asked the user to reinstall the app and compare network and device settings against working devices. No differences were found.
Checking back‑end and infrastructure
The team reviewed AWS security groups, Virtual Private Cloud (VPC) settings, firewall rules, IP‑whitelisting and SSL certificate versions. No misconfigurations were uncovered.
Routing traffic through Cloudflare
All API traffic was proxied through Cloudflare to protect against unknown carrier limitations. Although the back‑end domain loaded correctly in a browser after enabling the proxy, the mobile app still reported “No Internet Connection.
Root Cause Analysis
Carrier‑level objectable‑content filters
After eliminating app and server issues, our team at Ellocent further digging revealed that the carrier was silently filtering traffic to the domain. UK mobile networks apply content filters by default when they cannot verify that the subscriber is over 18. These network‑level filters manifest as TLS connection resets or time‑outs, so users see a generic “No Response” message instead of a clear block. The filters were originally applied when operators received no evidence of the subscriber’s age (e.g., pay‑as‑you‑go SIMs) but have since been extended to contract phones that could be used by minors. Removing these filters typically requires proof of age through a credit‑card charge or presenting a passport in an operator’s store.
Domain misclassification
The application’s API was hosted on a domain that had previously contained objectionable content and violated the BBFC Rule. Mobile carriers maintain filtering lists based on historical content and categorise domains accordingly. When the affected user accessed the API domain on mobile data, the carrier redirected HTTPS requests to an age‑verification page; if the user did not complete the verification, the network simply dropped TLS connections, returning a “Timed out” or “No Response” error. O2’s age‑restricted content guidance confirms that customers attempting to visit secure (HTTPS) objectionable sites may see a “Timed out” or “No Response” message. The Safer Shetland/Cyberfraudhub guide similarly notes that O2’s default age restrictions block 18+ websites and usually present a “Timed out” or “No Response” message.
Verification does not scale
When the user manually completed the carrier’s age‑verification page in a browser, the app began working on that SIM. However, this is not viable for a public consumer application because:
- Each new user would have to verify their age individually, which is burdensome and reduces sign-ups.
- Age-verification only works on one account; other users on the same network remain blocked.
- Many carriers require a credit-card or passport to disable the filter, creating friction. O2’s instructions state that users can remove age restrictions by logging into their account or visiting a store, but this is not practical for every candidate trying to access a job-search app.
Solution We Implemented
Migrating to a clean domain
We purchased a new .com domain that had no prior association with objectionable content and followed the BBFC Rules. The back‑end API and administrative panel were migrated to this neutral domain and placed behind Cloudflare. This ensured that the TLS certificate came from a trusted provider, concealed the origin server and reduced the risk of misclassification.
Choosing a trustworthy naming scheme
To avoid triggering content filters, our team adopted the following best practices:
Use widely recognised top‑level domains (e.g.,.com, .org) rather than obscure or novelty TLDs.
Avoid suspicious keywords in the domain or subdomain names (e.g., xxx, adult, admin), opting for neutral names like api.myrecruitapp.com or jobs.myapp.com.
Host a simple public landing page at the root domain with basic company information. Carriers often check the root page; having legitimate content helps reduce misclassification.
Before finalising a domain, we also checked how third‑party filtering services classified it. Tools such as Symantec’s Site Review (formerly Blue Coat) allow you to look up a URL and see its current WebPulse categorisation and, if necessary, request a recategorisation. The tool requires entering the URL, checking its category, and submitting a form suggesting a more appropriate category; updates typically take 24–48 hours. While Blue Coat’s tool only supports single URLs and there is no bulk‑submission option, using it during planning helped ensure the new domain was not flagged before launch.
Contacting the carrier and BBFC
Once the new domain was live, we contacted the affected carrier and the British Board of Film Classification (BBFC) to request reclassification of the old domain. O2’s guidance notes that incorrectly blocked sites can be reported to customer services. Providing proof that the domain now serves a recruitment platform helped expedite the removal from the objectionable content list. However, this process is far from seamless. There is no direct, clearly published route for developers to contact carriers or the BBFC; we could not find an online form or dedicated support channel on the operators’ websites. Instead, we submitted a generic support request and followed up via customer service lines. The carrier acknowledged the issue but noted that reclassifying a domain can take weeks. During this delay our client was losing business because users on certain networks still faced connection failures. This highlighted the need for a temporary fallback (such as a secondary domain) while waiting for carrier reclassification to take effect.
Subdomain fallback
We configured a secondary subdomain (e.g., backup.myrecruitapp.com) pointing to the same back‑end. Should a carrier misclassify the primary domain in the future, traffic can be quickly switched to the fallback domain without updating the mobile app.
Improved error handling
The mobile application was updated to distinguish between general connectivity issues and server unreachable errors. Using libraries such as @react‑native-community/netinfo, the app now checks internet connectivity and shows a meaningful message if the API is unreachable, suggesting that the user contact their carrier or use a different network.
Cross-network testing
Our QA process now includes testing on major UK carriers (O2, EE, Vodafone, Three, giffgaff) in addition to Wi‑Fi. This helps identify carrier‑specific blocking before deployment.
Outcomes and Lessons Learned
After migrating to the new domain and obtaining carrier whitelisting, our team at Ellocent ensured that users on all networks (including the previously affected giffgaff/O2 SIMs) could access the application without completing an age‑verification challenge. No further connectivity complaints have been received.
UK mobile networks apply default objectionable‑content filters when they cannot verify the subscriber’s age. These filters can silently block HTTPS requests to misclassified domains, returning “Timed out” or “No Response” messages. Removing the filter typically requires proof of age through a credit‑card or ID.
For consumer mobile applications, it is critical to select domain names carefully. Avoid reusing domains with any history of objectionable content, choose widely recognised TLDs and neutral names, and consider hosting a legitimate landing page to establish credibility.
When investigating network‑specific issues, do not assume all carriers behave identically. Test on multiple SIMs and use tools like the Open Rights Group’s blocking database to check whether domains are being filtered. Reach out to carriers or the BBFC if misclassification occurs.
Implement robust error handling in mobile apps so that users are informed when a connection problem may be due to carrier filtering rather than the internet connection. This improves user trust and reduces support requests.
Key Takeaways for CTOs and Chief Engineers
Carrier filters are an operational risk
Mobile carriers may block domains based on historical content or unknown subscriber age. Such filters manifest as silent TLS failures and can be difficult to detect when testing only on Wi‑Fi or a single network.
Domain hygiene matters
Carefully vet domains before associating them with critical APIs. Use neutral names and trusted TLDs and avoid reusing old domains with questionable histories.
Plan for misclassification
Maintain a fallback domain and monitor carrier blocking databases. Build processes for rapid domain migration and contact points within carriers or regulatory bodies.
User experience improvements
Clear error messages and connectivity checks in the app can guide users to solutions (e.g., try Wi‑Fi or contact their carrier) rather than leaving them frustrated.
Embed network testing into QA
Include SIMs from major carriers when validating new releases, especially when operating in markets with strict content filtering.
At Ellocent Labs, by incorporating these lessons into deployment checklists and QA processes, technical leaders can prevent similar issues from impacting user experience on a recruitment platform and maintain reliable connectivity across all networks.
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