Lebra is a pay as you go mobile company that will provide you with monthly invoices via email, showing the calls you have made to your Thai partner. You can also login to their web site and see what calls you have made this month.
No contracts, No hidden charges , No access codes. Purchase a sim card from their web site, stick it in your phone and start calling Thailand.
Calling Your Thai partner in Thailand from the UK
Landline 4p /min
Mobile 6p /min
http://www.lebara-mobile.co.uk
Calling Your Thai partner in Thailand from Australia
Mobile $ 0.03 /min
Landline $ 0.01 /min
http://www.lebara-mobile.com.au/en/
However you are contacting your Thai partner, make sure you can get the list of calls you make as the embassy are going to need them at the time of the final visa application.