you can try to use www.data24-7.com to find out which carrier a number is:

AT&T –                        cellnumber@txt.att.net 
Verizon –                     cellnumber@vtext.com or  phonenumber@vzwpix.com
T-Mobile –                    cellnumber@tmomail.net 
Sprint PCS -                  cellnumber@messaging.sprintpcs.com 
Virgin Mobile –               cellnumber@vmobl.com 
US Cellular –                 cellnumber@email.uscc.net 
Nextel -                      cellnumber@messaging.nextel.com 
Boost -                       cellnumber@myboostmobile.com 
Alltel –                      cellnumber@message.alltel.com 
Cincinnati Bell Wireless it’s cellnumber@gocbw.com
--eleven digit!
Metro Pcs 14352345762@mymetropcs.com