Experience has shown us that phone is not usually the best way to deal with support issues.
Email tickets are sent to a team of support staff and developers so someone can get to it quickly, phone calls only go to one person, usually the most junior members of staff.
Emails can be passed on and we can see the history of each ticket as it moves from person to person if it needs to be escalated.
Lastly, as our support team are often the same people as built your original project, phone support disrupts ongoing projects and impacts both cost and deadlines, both of which are always critical to customers.
If the phone is the best way to deal with an issue, be sure we will call you to resolve it.