I find that when my 9900 is set to 2g or 3g it will only switch from 3 to 2 if 3 disappears and not during a call. Thus if 3 provides a weak signal it'll be a bad line rather than switch automatically to a better 2g. If I am in the middle of a call it will cut off or the other person won't hear me despite there being a very good 2 signal. As a result my phone is on 2 all the time unless i need 3g.

Is this a phone issue or is my carrier to blame?

Thanks

Its the towers near you, most likely. I would try the 3G only option. If its not there, get the latest os from this forum and make sure you delete the vendor.xml after the OS package has been installed to the computer. After the os is installed, you will see the 3G only option.

Another reason for getting the latest OS is RF signal coding fixes.
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The 3g only would result in having no reception for most of the time. The 2g or 3g should resolve that but as I said it maintains a very weak 3g over a strong 2g so many of my calls are cutting off even while in 2or3.