Can receive but not send texts from my iPhone 4?

I got my iPhone today, changed from a Blackberry.

I had to change SIM cards as the iPhone requires a Micro SIM. So I changed SIM cards but kept the same number, that worked and the new micro SIM with my old number is in the iPhone.

I can receive texts from people, but for some reason I can't send back. The sending bar stops right before it gets to the other end.

Do I need to wait for something?
Do I need to do something with my old SIM?

Any advice is helpful

Go into settings and then under messages, make sure the setting for "Send as SMS when iMessage is not available" is turned on.

That should fix your problem.

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