Williams has top-flight experience from his time at Sunderland

The 30-year-old former England Under-21 international was a free agent after being released by Pools at the end of last season and had impressed on trial.

Williams, who can also operate anywhere across the back four or in midfield, has signed a one-year contract.

Meanwhile, former Sheffield United midfielder Kyle Nix, 21, has joined the Bantams on a monthly contract.

He has signed so that McCall can take a closer look at him before the transfer window closes at the end of August.

The arrival of Williams and Nix comes on the same day as that of striker Nathan Joynes, who has joined on a six-month loan from Barnsley.

But three other trialists - Simon Johnson, Kris Gate and Peter Leven - have left after failing to secure a deal, with Johnson joining Hereford on trial.