As I understand it, you enter the country from which you are departing on your final flight into Bangkok (Singapore) and that same flight number. BTW, are you on a single ticket for your SIN transit, or will you clear customs and check-in separately? If the latter, you will need to complete a Singapore digital arrival card.
As others have mentioned, you do not need a visa. That aside, to avoid mis-counting causing you a potential overstay if you decide to stay longer, 25 Apr to 25 May is 31 days (inclusive), not 30 days. Happy trails.
You’re well on the way. Now that you have joined the group today, next you should go to this group’s landing page, and type “retirement” into the search field (magnifying glass icon at the top of the page), hit enter, then spend a few hours of your time reading the thousands of helpful previous posts and comments. You should also thoroughly review the official Thai government visa website (links available in the above mentioned posts/comments). Good luck.
I use a Skype subscription (have done for many years). For USD $20 per month, I can call almost any phone (both landlines and mobile) on the planet. As an expat who also travels often, this has been an excellent investment.
PS - Bloody hell, you jinxed me! I just received an e-mail from Microsoft, to advise me that Skype is retiring in April.