I have several telephone sets at home. Must I take out several subscriptions?

No , For a traditional analogue phone set, not IP compatible, each set will have to have an ATA to be able to make calls via the Internet network of 3Starsnet but the ATAs need not necessarily be configured with different (subscription) numbers.  A IP phone (VoIP compatible) does not require a different number per set either.  A telephone set an actually be configured with the same number.  The result will be that all the telephones will ring at the same time when that number is dialled.