Currently on ADSL, if I change to True Fibre does it mean I can cancel the existing phone line? I don't need a home telephone.

Correct. However, since phone lines can be hard to get in some areas you might want to retain the line for "phone number" use only which only costs Bt107/month until you are sure the Fibre to Home works good for you. Just cancel the internet ADSL but say you want to retain the line for phone use...up to you... but as you said you don't need a home landline number.

A home phone number can have several uses.

If you have a multifunction scanner/printer/fax, you can use it for the rare occasion when you might want to fax something in country.

I also give my home phone number to people who I now I do not want calling me when I am out and about. Plus I have an answering machine on it which, obviously, let people leave a message and I can then determine if I want to return their call or not.

I have my cell phone reroute calls I do not answer to my home phone, again hoping to get a message to see if I need to call or do something.

Many True Hi-Speed fiber plans provide a free land line phone number directly from there fiber optic modem at no cost.