In a statement to Fortune, Verizon said the fee is intended to offset the cost it incurs in reducing “spam and dangerous text messages” while reducing the incidence of fraud. The company said “there is no Verizon fee to parents or schools,” and stated it was “working through plans” with Twilio and Remind to prevent fees from being charged to Remind’s educational users.

However, Remind said in a blog post, “Your Remind messages aren’t spam, but our attempts to work with Verizon to find a solution haven’t been successful.” Twilio didn’t respond to Fortune‘s request for comment, while Remind said its blog post laid out its position.

Remind said many of its users lack home broadband, smartphones, or unlimited data plans, making text messaging a critical outreach for schools and teachers to keep parents and students informed of assignments, school events, and other academic matters.

While Verizon described the fee as “very small,” Remind said it would grow the company’s cost elevenfold in supporting Verizon customers. None of the companies involved disclosed actual amounts. Remind does have paid plans, relied on by a small percentage of its users, which will continue to work with Verizon text messaging. Remind has over 31 million users, most relying on a tier that involves no payment for texting.

Verizon said Twilio manages 4.5 billion text messages a year over its network, and that Remind constitutes 1.6 billion of those.

Remind has advised its users with Verizon service to enable notifications over email and using native smartphone notifications by installing the Remind app for iOS or Android.