Firstly, find the portable phone's base unit (DECT). It's a black rectangle, a bit larger than a phone, it'll probably say Yealink on it and have three lights, like the pic below .
Reboot the base unit by unplugging the power lead from the DECT base and wait 10 seconds (all the lights should be off). Then plug the power lead back into the base and wait until all the lights are solidly on. They might flash on and off for a bit before becoming solid. While the base is repowering, remove the back of the portable phone and take out the battery, wait 10 seconds, and then put the battery back in. Wait a few minutes and then test the phone.