I have installed iPhone i OS4 Software on my iPhone however I still can't use MMS.

Reset the APNs within settings on the iPhone, as follows:

Settings
General
Network
Cellular data network
Reset Settings




If you're a Pay Monthly customer but are not on an iPhone tariff then you will need to change two APN settings. Under cellular Data tap the APN setting and change it to mobile.o2.co.uk and then under MMS tap the APN setting and change it to wap.o2.co.uk

If you're a Pay & Go customer then you will need to change two APN settings. Under Cellular Data tap the APN setting and change it to payandgo.o2.co.uk and then under MMS tap the APN setting and change it to payandgo.o2.co.uk