An important part of being a sponsor is proving that you are in a relationship with your Thai partner. Some of this evidence the embassy require can be provided by contact and communication between you both.
One of the problems we find as a company is that the sponsors, ie the boyfriend use cheap calling cards to telephone their Thai girlfriend, but the calling card will not provide a invoice or bill listing all of the times you called you Thai partner and the length of time you spoke.
The same is with normal pay as you go phones
With most pay as you go phone companies you have to write to the company and request a list of all the calls you have made. The company will then write back to you asking for payment. Often this process goes on for months before you finally receive the list of calls.
There are telephone companies that work as pay as you go, with cheap rate calling that will provide monthly invoices via email