Yes-Today

When studying in the UK, some innocent students would buy Pay as you Go (Pay & Go) SIM card like me...

Actually, you can get free SIM card from website, such as  http://www.freesimcards.org.uk/

As a O2 user, I would like to share my own experience with people who use O2 as well.

(I am not saying it is the most economic way to make a phone call in the UK as I do not use mobile very often)

Basically, I suggest O2 user that you register My O2, by doing so, you can get 10 free on-line texts every month and check your current balance and allowance via Internet.

When you top up, you can get 10% of all your Top Up's back every 3 months, please refer to O2 Reward

If you top up more than ￡10, you will get chance to earn some gifts, such as unlimited text within 24 hours, for more info. you can refer to Top-up Surprise. However, I have to say I did not gain much from this based on my own experience.

O2 provides many different Tariffs, you can choose one according to your needs and can also switch to another tariff at anytime.

Frankly speaking, I only switched tariff once so I cannot conclude any pattern how it runs.

My own experience is as follows:

My original tariff was "Talkalotmore" (i.e. when I top up ￡10, I get 50 UK off-peak (Peak times are Monday to Friday 7.30am-7.30pm) and weekend minutes to call standard UK mobiles and standard UK landlines from the UK). As cited in terms and conditions: "Any unused Talkalotmore call allowance will not roll over from one month to the next." I remembered that in the past when I topped up ￡10, my free allowance could only last for one month. However, till last time, I found the allowance was rolled over to the next. I'm not sure if it is normal, but it did happened twice in my case.  

When I topped up by ￡10 on 13rd of November, I got 50 min. allowance. At the same time, O2 also reminded me that I should top up by ￡10 or more before 13/12 to get my free talkalotmore min.

On 29/11, I changed my mind and switched to "Text & Call" tariff. (i.e. when I top up ￡10, I'll get 25 minutes and 25 texts anytime to any mobile and landline in the UK). However, O2 did not switch my tariff to "Text & Call" right away but only text me "I'm already on Text & Call."

At that time, I did not run out of my allowance (around more than 40 min. left). Furthermore, between 14/11 and 12/12 , I did not top up any more. On 13/12 (the deadline I have to top up or I would lose my allowance), I got a text from O2, saying that "You've now got 25 free text messages to use this month. Remember to top up at least ￡10 by 13 Jan to get your free allowance next month"

To sum up, I  topped up ￡10 and got some free allowance to use within 2 months. If I use the allowance properly, my top-up credit would not be deducted. 

Note. There is a big difference in the way of call charges between Taiwan and UK. If your carrier says that it will charge you 20p/ per min. That is, no matter you spend 1 second or 59 seconds in one call, you would be charged 20p. As the minimum unit they charge you is per minute, you should keep in mind that when you are calling someone, hang up  the phone before entering into voice box.