It happens automatically except when it doesn't, and their documentation does not provide the slightest clue how to tell whether it has or hasn't. I'm still desultorily pursuing a refund of £7.38 I was charged in September for a six-minute US-to-UK call (instead of the promised £0.18) because my phone wasn't latched to Toggle.

You can call Toggle Customer Support at +44 7438 646 322 to find out whether your phone is on the Toggle network. Prime your ears for some really impenetrably-accented English. (-: Toggle says they charge 9p/call for these calls now, but my account wasn't charged for the call I made a few days ago.

I'm not sure where you find the SIM menu on an Android phone as I never had to use it while in Europe and my Android phones are locked to Verizon in the States. I'd start by looking in System Settings.