First of all, if flying 121, the flight attendant isn't going to be "coming in on" me during a takeoff. Our cockpit doors are locked from the inside and can't be opened from the cabin side. I do however appreciate your question.
My takeoff briefing with the first officer covers three phases during the ground roll according to our company policy:
Below 80 knots --> abort for any reason (i.e. caution lights, warnings, including flight attendant call)
Between 80 knots and V1 --> abort for engine failure, engine fire, loss of directional control, or the airplane won't safely fly
Above V1 --> takeoff, handle as inflight emergency, accomplish QRH items and evaluate the situation.
During my brief with the flight attendant, I tell him/her that if he/she calls the cockpit during the takeoff, the takeoff will be aborted. This indicates to the flight attendant the seriousness of calling the cockpit during this phase.
That being said, the call to the cockpit will not cause a high-speed abort. In fact, I would not even answer the ding of the phone until we were safely in the air as we have more important things to focus on.
BL