While you have already compared settings on the phones, check out these settings that are often overlooked regarding the Internet connection, and not just the messaging portion.
Run through the list, and if it all matches, you should be all set.
From the standby screen, press the Menu Key.
Scroll to "Settings," press Select.
Scroll to "WebAccess," press Select.
Scroll to "Web Sessions," press Select.
Scroll to "Cingular MMS" press Menu.
(if not there select "new")
Scroll to "Edit," press Select.
Validate the following parameters:
Name: Cingular MMS
Service Type 1: HTTP
Gateway IP: 18.104.22.168
Port 1: 9201
Domain 1: leave blank
Service Type 2: WAP
Gateway IP 2: 0.0.0.0
Port 2: 0
Domain 2: leave blank
DNS 1: 0.0.0.0
DNS 2: 0.0.0.0
Time out: 5 minutes
CSD no 1: leave blank
Username: 1: leave blank
Password 1: leave blank
Speed (Bps) 9600
Line Type 1: ISDN
CSD no 2: leave blank
User Name 2: leave blank
Password 2 leave blank
Speed (Bps) 2: 9600
Line Type 2: ISDN
GPRS APN: wap.cingular
User Name: WAP@CINGULARGPRS.COM
Scroll to "Messaging," press Select.
Press the Menu key.
Scroll to "Inbox Setup," press Select.
Scroll to "MMS Message Setup," press Change.
Scroll to "Server Info," press Change.
Scroll to "MMS", press Menu key.
Scroll to and select "Edit".
Validate the following settings:
Service Name: MMS
Server Name: http://mmsc.cingular.com
Web Session Name: MMS