To get maximum throughput from txtNation's SMSC HTTP connection, it is important that you conduct your HTTP Posts in Parallel rather than Sequentially.
A good description of the difference between Parallel Processing and Sequential is as follows:
"Parallel programming involves the concurrent computation or simultaneous execution of processes or threads at the same time. While Sequential programming involves a consecutive and ordered execution of processes one after another."  *
In PHP Standard HTTP requests GET / POST tend to be implemented using cURL. For Parallel HTTP it is not suitable. There are a number of libraries available to do the job.
Our preference is Rolling cURL: https://github.com/chuyskywalker/rolling-curl
Curl_Multi is another option: http://www.php.net/manual/en/function.curl-multi-init.php
A good point of reference for beginers is: http://www.codediesel.com/php/parallel-curl-execution/
For speed of execution and reliability doing it with Perl would be better, however it's up to your developers to select the one they are most comfortable with.
How many concurrent HTTP connections should I launch?
You will need to establish as many concurrent HTTP connections as is required to achieve your required transactions per second.
This should be calculated on the fly by the application, as there are to many variables in play to make it hard set.
We recommend starting with 10 concurrent HTTP requests, then increase by increments of 2 based upon the statistics you are getting back each minute.
Please speak to txtNation if you believe you require more than 10 simultaneous connections.
*  Note not all of the information on that page is relevant.