ISLAMABAD - President Asif Ali Zardari on Wednesday telephoned PML-N Chief Nawaz Sharif as well as ANP Chief Asfandyar Wali Khan and invited them to attend the proposed Roundtable Conference being convened by him. The President on Tuesday had convened the Roundtable Conference to hold deliberations on the current challenges being faced by the country and invited leaders of various political parties to attend the proposed conference.