To Place a phase on hold click the 3 dots to access the phase menu and choose Edit.
Next choose the On Hold option to place the phase and any phases that have task dependencies on hold.
Visual indicators will show the phase is then on old.
When you're ready to take the phase off of hold then click edit phase again and turn the phase hold off.
Based on company configuration you may need to adjust the schedule when you take the phase off of hold. Or utilize the split functionality (here) to insert 0 hours for the number of weeks the phase was on hold which is completely optional, if you'd prefer to just update the schedule going forward for the remaining work.