Error 21 with barcleys number

[#1]

Hi I'm trying to reply back to a text message to verify my purchase with barclays but every time I do, it says  "can't send message with plusnet, error 21",  I figured might be because it's a premium rate number, so I switched my smart cap on my account to £10 and that didn't work, still getting this same error.   Any idea what is causing this?

[#2]

You cannot reply to short codes on Plusnet Mobile.

[#3]

Hi Welcome to the Forum

Is the number a five digit short text number if so PlusNet do not support short text numbers so you are unable to reply

There are many posts on here about the problem 

[#4]

I think you figured wrong - is it a short code number? If yes, PN don’t support them.