Something Decembersangel said gave me an idea. Smarty block all things not paid for, or not in your plan. As short code is out of plan, have you tried adding some credit to your account and see if that lets you send short codes?
Put say £5 on and test. Unlike everyone else, you don't lose that money either! It stays in your account forever/until used up and at any point you can apply it to your bill so effectively get it back. I tend to add £5 to my account to cover things like this.
Decembersangel 'Visual Voicemail' is something that has been around for decades yet barley no network in the UK uses it so its unsurprising not many people know about it. I think only the very over priced networks use it like O2 and maybe one other? Basically when you click on the voicemail button you get a list a bit like an email list of your voice mail, allowing you to ignore and not even listen to your messages and just select the ones you want to listen to. When it came out I thought I really wanted it, but then the reality of how vastly expensive it would be (changing to a mega expensive restrictive account) in combination with I get roughly 1 voicemail a year, and the "I want it" soon became "meh!".
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