How do i txt to short numbers eg. those numbers frm a pre-paid service?

[JulieSmart]

I'm looking to send a txt msg to a short number that is part of a pre-paid service but my msgs show as failed. This is suggestive that my a/c has blocks which require to be enabled in order for me to proceed. Can u advise how I gain access to allow me to send such txt msgs - thanx

[MI5]

@JulieSmart 

Do you have or have reached a spend cap?

Guide: Spend caps on O2