How to LEAVE without a telephone call?

[#1]

Okay, so ive had a gutful, one thing after another after another.
I dont want to phone as im certain i will end-up getting into an argument with the tele-sales advisor (or whoever).
I just to pay what is owed (less charges) and change at the quickest possible opportunity.
If I do it by post, there is the time factor to consider, plus the denial of recieving the post.
If I email, it will go unanswered.
What is the safeist option in that sense?!
MAC CODE please!!!

[#2]

Recorded delivery letter, with "time is of the essence" quoted in your request. 

[#3]

thank you HPsauce