Follow the steps below to enable MMS on iPhone 3G/3GS using Legacy from Cydia. You may follow our previous guide on how to enable MMS on iPhone EDGE/2G running on 3.1.2 firmware. Make sure that your iPhone 3.1.2 firmware is jailbroken. Mac users can follow our guide on how to jailbreak iPhone 2G 3.1.2 using Pwnage Tool and windows user can use Blackra1n to Jailbreak iPhone & iPod Touch with iPhone OS 3.1.2 firmware installed.
Steps to Fix MMS issue For T-Mobile USA iPhone Users

Step 1: After jailbreaking your iPhone 3.1.2 firmware, open “Cydia” on your iPhone.

Step 2: Search for Legacy

Step 3: Select TMobile US (the 1st option)

Step 4: Once installed Reset Network Settings (Settings> General> Reset> Reset network settings)

Step 5: Input Internet & MMS Settings.

Internet: Settings> General>Network>Data roaming set to on> Go to cellular data network> APN=
MMS: Settings> General>Network> Go to cellular data network> Go to MMS apn and type
MMSC is proxy is

Step 6: Select return and power cycle the phone

Step 7: Test Internet

Step 8: Test MMS. it should work now.

the resource

hope will work to everyone

See More: how to enable MMS on iPhone EDGE/2G