Question

Must или had to?l forgot to phone Sam yesterday. I ____ phone him today​

Answer 1

I forgot to phone Sam yesterday. I MUST phone him today.

had to используется в прошедшем времени, а так как у нас указано настоящее [today], то тут подходит только must.

Answer 2

In the given context, the correct phrase to use is "I must phone him today." This implies a sense of obligation or necessity to call Sam.