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That could be caused by a few things, but one of the likely culprits is when your phone is switching from LTE to HSPA (3G). Currently, none of the carriers in Canada offer voice service over their LTE networks, so when you receive a phone call or SMS, some phones won't switch back to HSPA fast enough, causing the call/msg to get lost. Maybe lost isn't the right word....but it doesn't notify you on your end.
Before calling TELUS, i might suggest making sure you have the latest OS. If you do, then give them a call to rule out network issue.