Are you mixing up membership and add-ons?
Membership lasts a month.
If you buy a membership while another one is active then the newly purchased membership "stacks" and only kicks in when the current membership month is up.
Buying a new membership does not prematurely terminate an existing membership.
Memberships never overlap.
Add-ons last a month from the date of purchase - or until you have used up the allowances that they offer.
If you buy one - say landline calls - and use up the allowance then you can buy another one.
If you used up the landline mins on your membership then you probably need to buy a landline calls add-on.
If you have used up the internet allowance on your membership then you probably need to buy an internet add-on.
If you find that you are using up specific allowances on a membership then you should check in case there is another type of membership that better suits your needs and usage profile.
All of this is a bit different to how other pay as you go operators work but it is all explained on the site.
