diff --git a/README.md b/README.md
index 7138142..1d07d03 100644
--- a/README.md
+++ b/README.md
@@ -53,6 +53,21 @@ accountManager.registerSingleDevice("myCode").then(result => {
 });
 ```
 
+##### CAPTCHA challenge requirement
+Sometimes the Signal server requires a captcha token for registering a new account. The registration fails with a captcha required error or with `402 Payment Required` error. In this case, you need to solve a CAPTCHA challenge.
+
+To get the token, got to [Captcha generation](https://signalcaptchas.org/registration/generate.html). After filling the captcha, the site doesn't show the token but redirects to a signalcaptcha:// url that contains the token. To see the URL you can open the browser developer tools (F12) before completing the captcha. Check the console/network tab in the developer tools for a redirect starting with `signalcaptcha://`. Everything after `signalcaptcha://` is the captcha token.
+
+You can pass the captcha token to `registerSingleDevice` function like so:
+```
+const captchaToken = "...";
+accountManager.requestSMSVerification(captchaToken).then(result => {
+  console.log("Sent verification code.");
+});
+```
+
+Note: Captcha token is valid for only a short period of time.
+
 ### Sending messages
 
 To send a message, connect a `MessageSender` instance to the Signal service:
diff --git a/src/AccountManager.js b/src/AccountManager.js
index a7a3c6c..2297b6c 100644
--- a/src/AccountManager.js
+++ b/src/AccountManager.js
@@ -45,8 +45,8 @@ class AccountManager extends EventTarget {
     return this.server.requestVerificationVoice(this.username);
   }
 
-  requestSMSVerification() {
-    return this.server.requestVerificationSMS(this.username);
+  requestSMSVerification(token) {
+    return this.server.requestVerificationSMS(this.username, token);
   }
 
   async encryptDeviceName(name, providedIdentityKey) {
diff --git a/src/WebAPI.js b/src/WebAPI.js
index 27bd0b9..2ebc380 100644
--- a/src/WebAPI.js
+++ b/src/WebAPI.js
@@ -616,11 +616,11 @@ function initialize({
       });
     }
 
-    function requestVerificationSMS(number) {
+    function requestVerificationSMS(number, token) {
       return _ajax({
         call: 'accounts',
         httpType: 'GET',
-        urlParameters: `/sms/code/${number}`,
+        urlParameters: `/sms/code/${number}${token ? `?captcha=${token}` : '' }`,
       });
     }