I am wiring a new phone line to my home office. I only have the one phone line coming in the Network interface. If I run Cat 5 cable to the outlet to were my router will be, do I need all the 4 pairs to that connection or can I branch wires from the cat 5 for my phone line.
Or do I have to run to separate Cat 5's? Old house, want to do a basic wire on it.

A Phone line only uses 1 pair of cables, the more lines you want the more pairs you need, if you want to add another point - unsure of where you are, but for Australia, techinically you need to have it at the junction box where the telephone line comes in from the street - since your doing it "incognito" you can just loop off you're existing phone point - though the phone company can tell if you've got multiple phones plugged in and may charge you accordingly for multiple points.

If you've got a cat5 cable running to a plate, and you are using 1 pair as phone, you can technically use 2 of the other 3 for a network connection to your router. Te pinouts I would use are as follows:

Blue/WhiteBlue - Phone (pins 2 and 3 in the phone connector)
OrangeWhite/Orange/GreenWhite/Green - Network (Pins 1, 2, 3, and 6, in that order). Pins 1 and 2 must be one pair, and 3 and 6 the other.

A network cable only uses pins 1, 2, 3, and 6, these days. This is tru for 100MB (and I believe gigabit) networks. If you're using an older 10MB network, it uses pins 3, 4, 5, and 6. Pins 3 and 6 are one pair, and 4 and 5 are the other.

I'm not sure if I understand your question. You have one telephone number. That line runs to your network interface device (NID). From the NID, you have older cat5 cable running to the room where the router is. As aidos said, you only need one pair from the 4 wire cat5. Use blue for the 1st telephone line, orange for the 2nd line, etc. Split the pair and punch the solid blue wire down into the solid blue slot and the other wire into its corresponding striped slot on the back of an RJ-11 connector. You will need a punch-down tool to do this. You can now plug your phone line into the RJ-11 jack.

I would use a whole house DSL splitter and use the blue pair for voice, and the brown pair for DSL.
Blue= first pair
Orange = second pair
Green = third pair
Brown = 4th pair
If you use wall plates, the screw terminals will work.