Mark ticket as responded manually

At present, the only way to have a ticket marked as responded to is to send an email.
We have a P1 SLA with a very short response time and the majority of these tickets are handled over the phone or in person.
It would be very useful if an agent can mark the ticket as having been responded to manually whilst they are on the call so that the response SLA is not breached whilst they are actually resolving the issue