This could be caused by many things.
To start off, it looks like you're sending the extension number as your FROM URI. Your provider might not like that, most providers require a true number to be sent as the FROM URI. In your case +255222211150 should be sent as the outbound FROM and Caller ID field. This - might - fix it.
The other thing if that doesn't fix it, your provider may require an account code or other string of digits before the number you are trying to dial, also they may require a full country code before it will accept the call. This is provider specific and you might have to look at the provider's documentation for requirements on outward dialing.
Third thing is that you may need to whitelist your external IP with the provider to make sure you are allowed to dial outbound from your system. Again, this is provider specific and can change depending on who you are dealing with.