Some network operators don't like automated messages, in that case you can set variable delay. Just type "8-20" as delay on Profile page and click Save. It will select a random delay between 8 and 20 seconds for each message.
You can turn on "Send next message in campaign only after current message is processed by network operator" option on Profile page for a better experience. If you didn't enable "Request a delivery report for each message you send (Only SMS)" option on Profile page, you can set delay to 0 to send messages faster when the previously mentioned option is enabled, but it is recommended to keep 2 sec delay for SMS if you have delivery reports enabled. If you still face issue where messages are getting Failed status or stuck in Queued status for a long time, then try to increase the delay between messages.
It is recommended to keep 2 sec minimum delay for SMS and 4 sec for MMS. For SMS if you are seeing more than 1 message while sending as shown in following screenshot then just multiply number of messages with 2 and set that as a min delay. So for 4 messages as shown in the screenshot, it is a good idea to set 8 (2 x 4) sec minimum delay.