My Siri-initiated timers are always done with my phone, probably 50 or more each week (work stuff). The only time I get a failure is when I release the side button too quickly. I've made certain the spoken feedback is enabled to reduce the risk of me making that mistake. (Settings > Siri > Siri Responses > Prefer Spoken Responses)
As for, "What time is it?"... Try activating Siri and only saying, "Time."