Using a BT Home Hub ?

The newer BT Home Hub's actually output three wifi networks.

1. Your typical "BTHOMEHUB2-xxxx"
2. One called "BTOPENZONE"
3. Another called "BTFON".

The first is for your own personal use, which you have the password to. The others are unlocked, and are to allow any BTOpenzone user to connect to. Basically, BT are trying to make a ton of public wifi hotspots.

Sounds like maybe your phone lost connection to the first, so automatically joined the second. It should be as simple as going into wifi settings and choosing your original wifi network.

To stop it doing so in future, click on the "BTOPENZONE" network in that same setting and click forget.