Thai girlfriend Mobile phone
Many of the new type of phones including the Iphone and Blackberrys it is possible to link them to a PC and download all of the received calls and print them off. This is a
great way for your girlfriend to get evidence showing that you talk on a regular basis.
Your girlfriend will also be able to download all text messages received and any calls she has made to you from Thailand.
Then you supplying phone bill from your phone company will strengthen this part of the application.
Your Mobile phone
You will also be able to do the same, though the embassy will still require evidence via bills. However if you do have this type of phone print off the information and put it together with your phone bulls
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.