When checking for the times when messages are actually delivered to a user’s mobile phone you cannot rely on the DLR (Delivery Receipt) data as DLRs cannot be used to understand when a message is actually delivered.
Note: In many messaging systems, message status reports or delivery reports (such as “Delivered” or “Read” receipts) are sent based on whether the message was successfully delivered to the recipient’s device, not whether the recipient has seen or acknowledged it. Thus, if the device has received the message, the delivery notification will usually be sent to the sender regardless of the DND (Do Not Disturb) mode.
Although Sinch always endeavours to send all messages as quickly as received, there are several reasons why a message may arrive late on the user’s mobile phone:
Mobile phone was switched off, was off-network, had no connection, or was in airplane mode during delivery attempts. Most mobile operators have exponential retry logic.
User had switched off or silenced do not disturb/notifications.
User was sent a multi-part message where the final part was delayed for some reason. The phone will wait until all parts arrive before the full message is displayed.
Operator couldn’t reach phone for some other reason due to a handset issue:
Poor Signal Strength: If the sender or receiver has a weak or unstable cellular signal, the message or parts of it may not be delivered.
Message Filtering or Blocking: Carriers and Aggregators may filter or block messages they suspect as spam or prevent sending if the recipient is on a blocked list.
Inter-Carrier Issues: Text messages between different carriers (for example, between different mobile service providers) can sometimes face delays or delivery failures due to interoperability issues.
Full Inbox: If the recipient’s SMS inbox is full, they may not receive new messages. (principally older devices, sim stored or pre-paid account out of balance)
SIM Card Issues: Problems with the SIM card, such as it being improperly seated, damaged, or inactive, can cause message transmission failures as well.
Roaming Restrictions: When a user is roaming internationally, there may be restrictions or additional charges that affect the ability to send or receive messages.
Blocked Numbers: If the recipient has blocked the sender’s number, the message will not be received.
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