Hi,
Indeed as already mentioned there is no way of converting Cat3 into Cat5 or Cat5e.
If fact, Cat3 is totally different than Cat5. HOWEVER, you may get similar speed using Cat3s as Cat5 under 100MB. Over it is almost impossible.
Also, you may be able to use ONLY 2 pairs in order to get it running. Normally, you may use wires 1,2,3 and 6 as regular RJ45 jack ethernet. Wires 4 and 5 (the Internal middle pair) is widely used as Phone line. Wire pair 7 and 8 are not used at all, SO, IN OTHER WORDS, you can do this:
Cat3 - use multiple pairs (2) in order to get it running as Ethernet;
Cat5 - use wires 1,2,3 and 6 as regular one and you can use inside the same cable wires 4,5,7 and 8 as 2nd Ethernet. NO PROBLEM. Beware that this is a workaround avoiding recabling. Unless you really need speeds over 100MB - which I sincerely doubt - you can do this.
Regarding the VoIP adapter, RJ11 is similar to RJ45. RJ11 has 4 cables, RJ45 has 8. Phisically speaking, the RJ11 male FITS into a RJ45 female normally, which is worldwide used by VoIP or regular phones. If you pick a regular RJ11, the internal pair is line one, both outside wires are line two.
Wish you all the best. Hope you don´t need to re-wire. Is there any possibility to use 802.11n? This may be also a solution. The max tested speed on most n Networks is around 8MB, which should be more than enough for your VoIP. VoIP normally uses something around 20 kB.
Hope this help you out.
LUIS CLOSS