plo009: Sorry i'm a bit confused, so put simply, first option can be done inside the phone box while installing another jackpoint requires wiring in the house which is more complicated an expensive?
The existing cabling you have that makes its way around the house from the garage is likely to be the standard 2-pair stuff used domestically. I forget its name sorry, I'm not a telco.
Anyway, only one pair is used traditionally for your phone - the second is there if you ever needed a second line installed, in which case you can tap off that second pair at any point to get a jack for your second line (say, a fax line on the very end of the chain in your office for example).
That second pair though, may not be connected through your jackpoints - often, it's just wrapped around the incoming/outgoing cable insulation inside the jack. you'd just need to join them together with the aforementioned gelcap joiners, then whereever you wanted to have a jack for your broadband, instead of joining, you'll just run the incoming pair from the previous jackpoint to the back of the BT jack.
To do this, inside the box in the garage, you'd connect this second pair in the outgoing internal wiring to the appropriate pair on the filter. The other pair would connect to the first pair in the internal wiring, and service the jacks you'd use for phones.