Can I run telephone cables and standard UTP CAT5/6 in the same conduit? Will it cause any interference or cross-talk?
You can mix UTP cable categories in the same conduit. The real reason not to is that it is harder to maintain because all the cable types exit the same place, and you need to separate them after they exit the conduit.
Be sure to maintain a fill ratio of no more than 40% to allow proper firestopping. There are calculators and tables for the National Electric Code to determine the number of cables you can fit in a particular size conduit to maintain the proper fill ratio.