You can't do that when messages get lodged "over-the-air", as
from a phone or modem.  The sender (originator) phone number is
put into the message by the SMSC (message centre) where it's
lodged.  There is no mechanism for passing a different
originator address to the SMSC.

However, you can include a "reply-to" address that's different
from the originator address.  This is a fairly recent feature,
so not all receiving handsets will handle it correctly.

You put this reply-to address into a UDH (user data header), and
turn the UDHI (indicator) bit on within the PDU-type octet.

If I wanted to send the message text "Please reply" to
+98765432109, and specify +12345678901 as the reply-to address,
then I'd try sending a PDU like:

0051000B918967452301F90000AA190A22080B912143658709F180B2CBE17919242FC3D979

See 3GPP 23.040, sections 9.2.3.24 and 9.2.3.24.10.1.17.

John