Carrier filtering is the blocking or silent dropping of SMS by an operator's anti-spam systems based on content patterns, sender behaviour or volume, often reported as a specific failure reason or not reported at all.
Operators run filters on inbound A2P traffic: URL shorteners, phishing patterns, banned words, unregistered senders, volume spikes, repeated identical texts. A filtered message may fail with a reason or, on some networks, be reported delivered and never shown. Filtering rules are local and change without notice.
Most filtering is avoidable: use a link on your own domain, keep codes free of links, use the sender type the country expects, do not send marketing as transactional, and warm up volume on new destinations.
At SMSMeteor: filtered failures are reported with the reason where the operator gives one; the content blocklist in the acceptable use policy exists to keep routes clean for everyone on them.