Nope, Twilio doesn't do fractional billing.

I can report back on how much it costs me to run this thing after a month. With 5 people coming and going average of 2 times per day each, I expect it is going to come out to around $0.30 per day x 30 days plus the $1/mo for the phone number -- so roughly $10/month... we'll see how close my guess is