Hi Frank - this is what you do. Go to http://www.cosmote.gr
On the Menu line just below the cosmote title click on the second option along - kivnto ( kineto)
On the page you go to, scroll down and towards the bottom on the right there is the word Paypal and logos of various forms of Bank Cards, click on this.
The page you are taken to offers various options for payment amount to select, further down the page you are offered various ways of paying, below this still ( an odd place for it to be introduced ) is the request for you mobile phone number to type in a box entitled "Parakaloume ( please ) ..... kivnto".
Beyond this point you are on your own - presumably tick the small box beneath this and the blue box bottom right.
The only reason this might not work is if your number is not officially Registered - in which case you must go again to the Cosmote office taking with you every piece of documentation you possess, perhaps assortment of home made cakes, flowers, possibly even a small item of jewellery. Yasas.