"Now according to MT8880 document, it should be happening but not in reality."

In reality, what exactly are you seeing on the IRQ pin?

"Can some body who has used MT8880 and MH88422, tell me what potential problem should i look into."

Since I have not used these particular devices, you'd probably prefer that I simply stay quiet, but I am compelled to ask "How do your country's tones fit within the the Call Progress Response frequency range shown in Figure 8 of the datasheet?" If they are off toward one of the extremes in the "May Accept" area, you might see some "chatter" on IRQ, in which case, you would need to add some software filtering.