In fact, it's pretty easy to modify some TV antennas for low-power transmission.
Too bad I trashed and threw away the old TV aerial that was on the roof when we bought the house
I have an old TV dish lying around - what could I do with that ...?
A TV antenna (VHF/UHF) is too big for cellular frequencies. They operate from 40mhz to 400mhz (+/- a few mhz). The driven element for 144mhz is 3.2' for a halfwave element. The driven element of a yagi for cellular reception would be measured in inches, especially if you were on one of the digital systems. I'm running the numbers off the top of my head, so don't yell if I'm off.
As for the dish, I *think* they're only good for 2.4ghz and higher. Lots of hams make use of them. I've seen the plans for antennas using them and they're pretty straightforward. However, my interest is in the lower bands, so I haven't done anything with the spare dish I have at home.
As for attaching a yagi to a cellphone, it should be pretty straightforward if your phone has a port for an external antenna, you can get the appropriate connector, and connect it to coax that's connected to your antenna. That said, you'd look pretty funny with your tiny flipphone and an antenna connected via a couple feet of cable.
Chris