I was trying to replace the sender address with an alphanumeric text by adding a UDH to the PDU string? Unfortunately this didn't work. Maybe because the spoofed address is being replaced by the SMSC?
From a normal mobile phone of mobile modem you cannot change the sender address via a SMS-SUBMIT PDU. The SMSC will change the sender address to your the ME address (mobile modem or mobile phone).
You can only change the sender address if you have a direct connection to an SMSC via HTTP, SMPP or SMTP for example depending on the SMSC service provider.