Ting Wireless is a US-based cell company that resells Sprint wireless services. www.ting.com
Ting’s service does not require a contract and for the needs of Nightscout connectivity, it is incredibly economical (approximately $10/month with taxes).
Ting supports various phones that can be used with Nightscout. The most popular phone has been the Motorola Moto G. Check with Ting before purchasing a specific phone to verify it works on their network.
Ting sells the MotoG phone for less than $100, which is one of the most inexpensive compatible uploader phones you can find.
Ting Wireless maintains a FAQ about Nightscout.