Hi guys, Im using a 3GS from Vivo (Brazil) and im getting into some weird problem while sending MMS.

If I go to SMS.App > Compose the message > attach a picture > Press Send, I get a "your message exceeds the 300k limit and can not be sent" message. If I go to the Camera Roll, select a pic and create a MMS from there, everything works flawlessly. I asked a friend that uses a 3GS on Rogers (Canada) and he can send pictures direclty from the SMS.App, so it looks like a limitation from my operator.

Anyway, does anyone knows how / where to change that limit? Btw, I do have official MMS from my operator, if that matters.

Are you able to get to the Cellular Data Network screen from Settings -> Network? Depending on your .IPCC or MobileConfig, you should have access to that and in that menu is the option you're looking for, MMS Max Message Size. For carriers that have a 300k limit, you should set it to the value below.

MMS Max Message Size = 307200

If you cannot access the Cellular Data Network screen to manually enter this, do a quick search on this forum or elsewhere for a .IPCC or MobileConfig for your carrier that allows editing the CDN screen manually -- it's a tremendous help. I believe you could find one at help.BenM.at – Mobileconfigs and Tethering for your iPhone | BenM.at but I haven't checked specifically for your carrier, YMMV.

I have been looking up Vivo's carrier bundle, max message size is set to 614400, double the value european carriers allow. If your iPhone is jailbroken and OpenSSH installed, give this one a shot.
SFTP into /System/Library/Carrier Bundles, open the Vivo bundle and edit carrier.plist. set the value for max message size to 307200.
Save, restart the iPhone and check it out.

I wont recommend to use mobileconfigs here at all.

You are on a legit iPhone contract with Vivo, right?

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iPhone 3GS factory unlocked (3.1.2; Blackra1n, Cydia, OpenSSH, custom 3.0 ipcc file (no sig), tethering hack by Dev team, 32 GB)
iPhone 3G (3.0.1; Redsn0w 0.8, Cydia, OpenSSH) 16 GB