This is an excellent question. If you are on a pay by the day plan and someone calls or messages you, your service is considered used for that day and you'll be charged your daily rate, even if you do not respond to the message or answer the call.
We do also have some really great Monthly 4G plans that may fit your needs - you can get a specific number of minutes, messages and data for one rate and not have to think about whether you've received a call or message that day. You'll just know. You can learn more about our Monthly 4G plans here: http://prepaid-phones.t-mobile.com/monthly-4g-plans
Thanks for your response. Another question has come up: Is it possible to change paybyday plans from one day to another, so e.g. normally being on the 1$ plan but switching over to the 3$ plan if i need the internet? Also, if I don't have any money on my account and someone calls me, will my credit card automatically be charged or will i in this case not be able to receive the call?
I can answer the first part:
You can switch from one day to another, but I strongly suggest you first switch to PayAsYouGo before midnight, and then the next day switch to your new plan (or at least after midnight). Otherwise you will pay for both plans the day you switch.
If you don't have any money on your account, and don't have a payed plan going, I'm pretty your phone can't be used for anything but SOS calls. You will not be able to call or recive calls or messages. Your credit card should not get chrages automatically from this.