Answer

If there is a fear of Bismillah being disrespected then it will be permissible to use the number 786. However, the practice of substituting Bismillah with the number 786 is not established from the Quran or the Ahadith. This was the practice of some of our pious predecessors. It is not a sunnah practice nor does it hold any virtue.

Only Allah Knows Best,

Mohammed Tosir Miah