Is it possible to install an App on WP 10 or 8.1 from a source other than Microsoft 'Store'?

I would like to use a utility that I have installed on my PC that allows me to send SMS via internet using my Phones mobile account. I can receive SMS' fairly easily out in the countryside where I live but sending is difficult without making a phone call from my landline to my mobile phone to get it freshly connected to the tower.
thanks
Rollo

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