There are many reports from users in your area and on your carrier that experience the same. Some have escalated through their mobile service provider and been told it is network congestion, but the "blame" shifts back and forth between the carrier(s) and RIM.
To get more attention to the matter, I recommend you ring up your mobile service provider and lodge a formal support ticket. The more of those that are opened, the more likely the issue shall receive the attention it requires.
Occam's Razor nearly always applies when troubleshooting technology issues!
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