Sometimes settings get changed when people look through the
menus to see what's there.

Another possibility that's occurred to me is that the OP might
be trying unsuccessfully to reply to messages from a certain
individual.

If Carsten can't reply, but can submit a newly generated
message, this inevitably means that the other party has "reply
path" turned on in their message settings.

The reply path mechanism is supposed to allow someone receiving
your message to make a single reverse-charge reply.  But no
carrier that I'm aware of implements it properly, and a certain
"do nothing" configuration can lead to the following problem.

The sender of the original message turns on the "reply path" bit
and their SMSC is supposed to carry that setting on to the
recipient if and only if they support the mechanism.  Then when
the recipient replies, their phone uses the original sender's
SMSC to submit the reply (not the SMSC configured in their
phone).  And that's the point where an incorrectly configured
reply path mechanism fails - when that SMSC rejects the direct
lodgement attempt from a device on another network that's been
invited to use it.

John

Not according to Optus at: http://tinyurl.com/h8uvl

That says: "Check that your SMSC number is correct. You can
locate your SMSC using your handset guide.  The SMSC or MCN
(Message Centre Number) needs to be set to the number
+61411990001."

John

There are a number of different SMSCes, just because yours is 33 doesnt mean
his should be too