In addition to Yvonne’s very good advice, clean internal contacts beginning with those of the battery (both on the battery and in the socket). This includes those of SIM and any SD card. Use a cotton bud and high-percentage (medical use) alcohol or isopropyl alcohol if you can get it.
If you are using a SD card formatted as removable / portable storage, try removing it (turn the phone off and remove the battery first).
If you have a SD card formatted as extension of internal memory, read this.
Lastly, take precautions against humidity and condensation. Avoid taking the phone from dry air-conditioned space out into warm humid environment without precautions.
Consider using an air-tight plastic pocket with a sachet of silical gel, and a Bluetooth headset capable of making and receiving calls.
When in air-conditioned space it’s probably a good idea to keep the phone as warm as possible, for example in clothing pockets, so it’s less cold when you go out and will receive less condensation.