For Visa purposes it will be fine to use the one you already have, but we do require teachers to take the free JuniorTEFL program that MEF provides.
The program is designed to be more practical and give you a heads up for training, first time visiting a school, different cultural differences, using our materials, standard lesson set up, things to remember, etc.
We have tied this in as part of our training. We just had a teacher recently, who already had their TEFL certification, take the JuniorTEFL course and not only did they feel it was helpful they also asked if they would be able to reference parts of the course to be able to use again later.
So, yes, we do ask that even teachers who have a different TEFL certification also do the TEFL program that is provided by MEF.
We are also a team, always trying to improve all parts of MEF and what we do. If there is an element of the TEFL you have already taken that you feel is missing in the JuniorTEFL program, we definitely welcome the support and feedback as we try to help you and the other teachers be as prepared as possible before going overseas.