Are you using the t/p line to the office simply for dial up connections or is it also used for voice calls? I got the impression it was only used for dial up and if this is the case you could use Puta's suggestion of a router to direct your BB connection to as many PC's as you like and the filter on the line would be used to take your personal calls. If you need both a personal and a business voice call line then, yes, you will need two t/p lines.
DHCP (Dynamic Host Configuration Protocol). Well you did ask! It is simply a method of allocating networking resources to all the PC's in a workgroup. DHCP controls the way the PC's link to each other and to the internet. If you want some really heavy reading, have a look at this.