Any harm in leaving T-Mobile UK Sim Cards in phones until I top them up in UK in Aug?

[gmmour]

In Greece, for example, all prepaid cards have to be activated as follows:
Insert the card in the phone and power on.
Call any billable phone number (i.e. no emergency number or free cc number) and wait until you listen a welcome message like "welcome to XX prepaid, your phone is now activated and you can make and receive calls" on your phone, then you can hang up before the call is actually connected. But you have to listen through the whole welcome message. Some opertors also send you a welcome SMS after that, but the SIM is activated right at the moment the welcome message ends.

Before activation, the card is officialy inactive even though the phone registers with the home network, doesn't accept or make calls or SMS and thus does not count towards the expiration date. This first call has to be made in the home network for the welcome message to be played and the card to be activated, i.e. you can't activate abroad on a roaming network...

Many greek carriers instruct their employees on their shops to activate all prepaid SIM cards unless the customers doesn't want it. If you buy the SIM card from a news-stand or a kiosk of course they won't care to activate your card unless you ask them to (and they know how to do it, I witnessed a Pakistani guy and my grandma asking the local kiosk lady to top his phone up).