In this post, by "SMS Gateway" I mean a company that receives SMS from
myself (usually over a server to server SMPP socket connection), and
then routes it to various carriers.

Here goes with my question. A few months back, I was told it may be
unreliable for non-US SMS gateways to send SMS into the US (where I
reside) i.e. the SMS sometimes never get delivered.

If this is the case, I would be very interested to know the EXACT
technical reason for this. Many voice calls originate from outside the
US - when my mom phones me from England (a mobile to mobile call), she
gets through, and connects EVERY time. Why not SMS?