Your Cat 5 ethernet cable is only using two pairs to carry the data traffic to and from the modem.
That leaves two pair spare.
You can use one of those pairs to carry the voice traffic. Telephony only uses one pair of wires.
I have an extra cat5 cable running from my comcast modem (internet and phone) in my study room to my family room across my house. Can I use this cat5 to connect a phone set in my family room to the modem, so I do not have to run a twisted pair over the house?