You can't - but you should be able to.

You can get a text message through with very little signal strength. The phone will generally also keep trying until the message gets through

There have been lots of cases of people lost in mountains being rescued because a text message to friends got through when a voice call couldn't. There was even a case of boat that sunk in the Caribbean who sent a text message to friends in the UK to call the coastguard.

You can also send a txt message if you have a single finger still working.