Firefox Relay, a Mozilla service designed to hide your “real” email address by giving you virtual ones to hand out, is expanding to offer virtual phone numbers. in a blog post Mozilla product manager Tony Amaral-Cinotto explains that the relay service generates a phone number for you to give out to companies if you suspect they might use it to send you spam messages in the future, or if you think they might share it with others who will.

The idea is that handing out this alternative phone number makes it easier to block spam phone calls or texts in the future. You can either block all calls or texts sent to your relay number, or just block specific contacts. Importantly it lets you keep your “real” phone number private, which is something you might want to consider if it’s a number you use to receive sensitive information like two-step verification codes via SMS.

“How many times have you shared your number without giving it much thought?” Mozilla’s blog post asks. “When you sign up and accept the terms of service, you’re also agreeing to share your personal information like your phone number with those companies — plus all their third-party partners.” It notes that sharing your number widely like this increases the chances of it leaking, and ending up on a spam caller list.

Once you’re signed up, the Firefox phone number masking service offers 50 minutes of incoming calls and 75 text messages a month. Mozilla’s blog post notes that you can use its service to reply to texts from your last sender, but that you can’t place outgoing calls or send texts to whoever you like (though the organization says it’s exploring offering these as features in the future) .

Unlike Firefox Relay’s existing email relay feature, which lets you generate multiple email addresses that forward on to your real account, the phone number service only gives you access to one relay phone number, and notes that this can’t be changed once it’s chosen. The phone number masking service is also more expensive at $4.99 a month (or $3.99 a month when paid annually), while the email service offers a choice between a free tier and a premium tier costing $1.99 a month ($0.99 a month when paid annually) . Finally, the new phone number feature is exclusive to the US and Canada for now, but includes access to email masking as part of the same subscription.

Firefox’s blog post doesn’t mention using any form of encryption on the text messages that pass through its relay service, which isn’t exactly surprising given SMS isn’t an encrypted protocol. We’ve followed up with the company for confirmation, but it’s worth bearing in mind if you were thinking of using Firefox Relay for receiving sensitive private or confidential texts.

If you’re interested, you can sign up at relay.firefox.com