Rise will work across the 118 118 business, focusing especially on its directory assistance text messages for consumers, and on its mobile marketing service Txt//Ad. This service was launched recently as a partnership with the search company Miva.
The agency's appointment will not affect other agencies on the 118 118 roster. These include OMD UK and Naked Communications.
William Ostrom, the commercial director at The Number, said: "The appointment of Rise will make 118 118 even more relevant and useful to millions of people."
Rise also handles communications planning for brands including The Macallan whisky, BT, AOL and Reckitt Benckiser.