To do the same thing Google was doing that year when they launched GOOG-411:

Using the interactive phone calls to train their speech recognition systems, so that they could eventually use what they learned to develop things like Google Assistant, Cortana, Windows Voice Recognition, etc.

Tellme was taking 2 billion calls a year when they were acquired. They had all the training data Microsoft could want to compete with Google in that area.

> In early 2012, Microsoft divested itself of Tellme Networks' interactive voice response (IVR) service and the majority of its employees to [24]7 Inc. The service was moved to a non-toll-free number.

https://en.wikipedia.org/wiki/Tellme_Networks