Why SMS Delayed?
SMS may be delayed due to your mobile phone is located outside of network’s coverage area. Driving at speed above about 50km/h, it will also cause the SMS delay.
Sending SMS during heavy traffic period will make your messages delivery delay. Those periods create heavy traffic congestion on the network carrier and caused your messages arrival delay.
If your inbox full of messages, messages application is frozen, or devices has been turned off, they will also negatively impact the messages delivery.
Each of the telco always prioritize their own traffic. When sender and receiver are using the different networks, delivery of SMS messages may have the problem of delay.
Why SMS Not Delivered?
Telco block/filter divided into P2P and A2P messages. P2P messages are sent between individuals. These messages are likely to be delivered without being filter. On the other hand, A2P messages are bulk send application. Telco can filter and block the messages that contain certain keywords. This depends on the specific of each individual’s telco.
Delivery Time Expired
SMS messages have a waiting period for the phone come back. During this waiting period, if the phone in switching off or not in the coverage, the SMS messages will expire.
Number is in Roaming
When recipients currently are not in the country and the device turn to roaming, the SMS messages also not guaranteed able to deliver to them.
Ported Phone Number Issue
Sometimes, there is a problem between telco (network) and phone number (sender and recipient) that caused the messages to get lost. Network tries to deliver the messages, but it finds no synergy between network and number.