After an agent makes a phone call inside an Open or New ticket the ticket gets automatically resolved as the phone call is ended. Some customers do not like this so if you are like them here is an example of automated rules which can prevent this from happening. First of all create some tag called for example Reopen call and then create similar rules like the following ones in order to achieve your exact scenario.
Rule nr. 1 should be something like this:
Rule nr. 2 should look something like this:
Basically these rules would work like this - when a call is started in a ticket the ticket gets assigned "Reopen call" tag and when the call is ended and automatically resolved the 2nd rule kicks in and transfers the ticket to the same department so it effectively opens the ticket again and it removed the tag. Set up like this has also one advantage, your agent could remove the "Reopen call" tag during the call and if this tag is removed the call will stay in "Resolved" status.