2-way SMS

People,

We have a requirement to allow 2 way SMS messages via our software.

I can see how 2-way SMS would work with a bank of different phone numbers,
and a bit of software deciding which messages should be sent from which
numbers, but I've been told by one provider that they can offer this service
with *one* phone number because, apparently, the GSM network sends a unique
ID with each SMS message. When the user of the handset replies to the
message, this unique ID is sent along with the message back to the GSM
gateway (eg, negating the need for several different telephone numbers).
The unique ID is *not* something the user has to type into their phone, it's
simply supported by GSM (apparently).

The thing is, that everyone else I've spoken to tells me that the above is
*not* possible. And that the only way to achieve 2-way messaging via
software would be either to have a bank of phone numbers, or to simply match
the incoming message with the phone number of any previously sent outgoing
messages (which is not ideal, as this won't work if two different people
send two different messages to the same mobile, which then replies).

Can anyone who knows slightly more about the technology of GSM give me their
take on the above? Does the GSM network allow some kind of unique ID to be
sent, and received on replies from handsets? (without the user having to
type it manually of course). Could someone point me at some GSM spec which
shows whether or not this mythical unique ID exists?! - I'm pretty sure it
can't be done, but the provider who say they can offer the above with a
single number appear to be very convinced they can... I don't want to waist
any more time on development if they're talking out of their bottom!

Many thanks,
A.