Hi venkat. Please refer to the previous you posted, Why does the ringer volume automatically go down on the Lenovo A6000?
Let's discuss it there. :)
it is happening randomly, I am increasing manually, but after 5 to 10 minutes ringer and notification volume decreasing to 25% level.
Does this happen only on the start of the connection? If it does, then it may be normal. If it happens randomly, check if volume down key is stuck, it may be the reason behind that.