diff --git a/app/src/main/cpp/baresip.c b/app/src/main/cpp/baresip.c
index a56a721c..7265f5cc 100644
--- a/app/src/main/cpp/baresip.c
+++ b/app/src/main/cpp/baresip.c
@@ -981,6 +981,19 @@ JNIEXPORT jint JNICALL Java_com_tutpro_baresip_Api_account_1set_1rtcp_1mux(
     return account_set_rtcp_mux((struct account *)acc, value);
 }
 
+JNIEXPORT jint JNICALL Java_com_tutpro_baresip_Api_account_1rel100_1mode(
+        JNIEnv *env, jobject thiz, jlong acc)
+{
+    return account_rel100_mode((struct account *)acc);
+}
+
+JNIEXPORT jint JNICALL Java_com_tutpro_baresip_Api_account_1set_1rel100_1mode(
+        JNIEnv *env, jobject thiz, jlong acc, jint jMode)
+{
+    const uint32_t mode = (uint32_t)jMode;
+    return account_set_rel100_mode((struct account *)acc, mode);
+}
+
 JNIEXPORT jint JNICALL Java_com_tutpro_baresip_Api_account_1dtmfmode(
         JNIEnv *env, jobject thiz, jlong acc)
 {
diff --git a/app/src/main/kotlin/com/tutpro/baresip/Account.kt b/app/src/main/kotlin/com/tutpro/baresip/Account.kt
index ba1fee5c..c374b489 100644
--- a/app/src/main/kotlin/com/tutpro/baresip/Account.kt
+++ b/app/src/main/kotlin/com/tutpro/baresip/Account.kt
@@ -25,6 +25,7 @@ class Account(val accp: Long) {
     var configuredRegInt = REGISTRATION_INTERVAL
     var mediaEnc = Api.account_mediaenc(accp)
     var rtcpMux = Api.account_rtcp_mux(accp)
+    var rel100Mode = Api.account_rel100_mode(accp)
     var dtmfMode = Api.account_dtmfmode(accp)
     var answerMode = Api.account_answermode(accp)
     var autoRedirect = Api.account_sip_autoredirect(accp)
@@ -123,7 +124,12 @@ class Account(val accp: Long) {
         if (mediaEnc != "") res += ";mediaenc=${mediaEnc}"
 
         if (rtcpMux)
-             res += ";rtcp_mux=yes"
+            res += ";rtcp_mux=yes"
+
+        res += if (rel100Mode == Api.REL100_ENABLED)
+            ";100rel=yes"
+        else
+            ";100rel=no"
 
         res = if (vmUri == "")
             "$res;mwi=no"
@@ -136,7 +142,7 @@ class Account(val accp: Long) {
         if (autoRedirect)
             res += ";sip_autoredirect=yes"
 
-        res += ";ptime=20;regint=${regint};regq=0.5;pubint=0;call_transfer=yes;100rel=no"
+        res += ";ptime=20;regint=${regint};regq=0.5;pubint=0;call_transfer=yes"
 
         var extra = ""
 
diff --git a/app/src/main/kotlin/com/tutpro/baresip/AccountActivity.kt b/app/src/main/kotlin/com/tutpro/baresip/AccountActivity.kt
index df00af78..0fe4f4c2 100644
--- a/app/src/main/kotlin/com/tutpro/baresip/AccountActivity.kt
+++ b/app/src/main/kotlin/com/tutpro/baresip/AccountActivity.kt
@@ -45,6 +45,7 @@ class AccountActivity : AppCompatActivity() {
     private lateinit var mediaEnc: String
     private lateinit var mediaEncSpinner: Spinner
     private lateinit var rtcpCheck: CheckBox
+    private lateinit var rel100Check: CheckBox
     private var dtmfMode = Api.DTMFMODE_RTP_EVENT
     private lateinit var dtmfModeSpinner: Spinner
     private var answerMode = Api.ANSWERMODE_MANUAL
@@ -95,6 +96,7 @@ class AccountActivity : AppCompatActivity() {
         stunPass = binding.StunPass
         mediaEncSpinner = binding.mediaEncSpinner
         rtcpCheck = binding.RtcpMux
+        rel100Check = binding.Rel100
         dtmfModeSpinner = binding.dtmfModeSpinner
         answerModeSpinner = binding.answerModeSpinner
         redirectModeSpinner = binding.redirectModeSpinner
@@ -191,7 +193,12 @@ class AccountActivity : AppCompatActivity() {
                                     if (text.isNotEmpty())
                                         acc.stunServer = text
                                 "rtcp-mux" ->
-                                    acc.rtcpMux = text == "yes"
+                                    it cacc.rtcpMux = text == "yes"
+                                "100rel-mode" ->
+                                    acc.rel100Mode = if (text == "yes")
+                                            Api.REL100_ENABLED
+                                        else
+                                            Api.REL100_DISABLED
                                 "dtmf-mode" ->
                                     if (text in arrayOf("rtp-event", "sip-info")) {
                                         acc.dtmfMode = if (text == "rtp-event")
@@ -307,6 +314,8 @@ class AccountActivity : AppCompatActivity() {
 
         rtcpCheck.isChecked = acc.rtcpMux
 
+        rel100Check.isChecked = acc.rel100Mode == Api.REL100_ENABLED
+
         dtmfMode = acc.dtmfMode
         val dtmfModeKeys = arrayListOf(Api.DTMFMODE_RTP_EVENT, Api.DTMFMODE_SIP_INFO)
         val dtmfModeVals = arrayListOf(getString(R.string.dtmf_inband), getString(R.string.dtmf_info))
@@ -620,6 +629,16 @@ class AccountActivity : AppCompatActivity() {
                         Log.e(TAG, "Setting of account_rtc_mux failed")
                     }
 
+                if (rel100Check.isChecked != (acc.rel100Mode == Api.REL100_ENABLED)) {
+                    val mode = if (rel100Check.isChecked) Api.REL100_ENABLED else Api.REL100_DISABLED
+                    if (Api.account_set_rel100_mode(acc.accp, mode) == 0) {
+                        acc.rel100Mode = Api.account_rel100_mode(acc.accp)
+                        Log.d(TAG, "New rel100Mode is ${acc.rel100Mode}")
+                    } else {
+                        Log.e(TAG, "Setting of account_rel100Mode failed")
+                    }
+                }
+
                 if (dtmfMode != acc.dtmfMode) {
                     if (Api.account_set_dtmfmode(acc.accp, dtmfMode) == 0) {
                         acc.dtmfMode = Api.account_dtmfmode(acc.accp)
@@ -799,10 +818,10 @@ class AccountActivity : AppCompatActivity() {
                 getString(R.string.media_encryption_help)
             )
         }
-        binding.RtcpMuxTitle.setOnClickListener {
+        binding.Rel100Title.setOnClickListener {
             Utils.alertView(
-                this, getString(R.string.rtcp_mux),
-                getString(R.string.rtcp_mux_help)
+                this, getString(R.string.rel_100),
+                getString(R.string.rel_100_help)
             )
         }
         binding.DtmfModeTitle.setOnClickListener {
diff --git a/app/src/main/kotlin/com/tutpro/baresip/Api.kt b/app/src/main/kotlin/com/tutpro/baresip/Api.kt
index fec2944f..5207af16 100644
--- a/app/src/main/kotlin/com/tutpro/baresip/Api.kt
+++ b/app/src/main/kotlin/com/tutpro/baresip/Api.kt
@@ -8,6 +8,9 @@ object Api {
     const val ANSWERMODE_AUTO = 2
     const val DTMFMODE_RTP_EVENT = 0
     const val DTMFMODE_SIP_INFO = 1
+    const val REL100_DISABLED = 0
+    const val REL100_ENABLED = 1
+    // const cal REL100_REQUIRED = 2
 
     const val SDP_INACTIVE = 0
     const val SDP_RECVONLY = 1
@@ -48,6 +51,8 @@ object Api {
     external fun account_set_answermode(acc: Long, mode: Int): Int
     external fun account_sip_autoredirect(acc: Long): Boolean
     external fun account_set_sip_autoredirect(acc: Long, allow: Boolean)
+    external fun account_rel100_mode(acc: Long): Int
+    external fun account_set_rel100_mode(acc: Long, mode: Int): Int
     external fun account_rtcp_mux(acc: Long): Boolean
     external fun account_set_rtcp_mux(acc: Long, value: Boolean): Int
     external fun account_dtmfmode(acc: Long): Int
diff --git a/app/src/main/res/layout/activity_account.xml b/app/src/main/res/layout/activity_account.xml
index 73715109..70a39ac3 100644
--- a/app/src/main/res/layout/activity_account.xml
+++ b/app/src/main/res/layout/activity_account.xml
@@ -336,11 +336,37 @@
             </CheckBox>
         </RelativeLayout>
 
+        <RelativeLayout
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:paddingTop="0dp"
+            android:orientation="horizontal" >
+            <TextView
+                android:id="@+id/Rel100Title"
+                android:layout_width="wrap_content"
+                android:layout_height="wrap_content"
+                android:layout_alignParentStart="true"
+                android:layout_centerVertical="true"
+                android:layout_toStartOf="@id/Rel100"
+                android:textSize="18sp"
+                android:text="@string/rel_100" >
+            </TextView>
+            <CheckBox
+                android:id="@+id/Rel100"
+                android:layout_width="wrap_content"
+                android:layout_height="wrap_content"
+                android:layout_alignParentEnd="true"
+                android:layout_gravity="end"
+                android:layout_centerVertical="true"
+                android:checked="false" >
+            </CheckBox>
+        </RelativeLayout>
+
         <TextView
             android:id="@+id/DtmfModeTitle"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            android:paddingTop="10dp"
+            android:paddingTop="8dp"
             android:textSize="18sp"
             android:text="@string/dtmf_mode" >
         </TextView>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index d6dbd761..a914faae 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -170,6 +170,8 @@
     <string name="prefer_ipv6_media_help">If checked, offer to use IPv6 media protocol (if available) when media protocol of peer cannot be automatically determined.</string>
     <string name="rtcp_mux">RTCP Multiplexing</string>
     <string name="rtcp_mux_help">If checked, RTP and RTCP packets are multiplexed on a single port (RFC 5761).</string>
+    <string name="rel_100">Reliable Provisional Responses</string>
+    <string name="rel_100_help">If checked, indicate support for reliable provisional responses.</string>
     <string name="dtmf_mode">DTMF Mode</string>
     <string name="dtmf_mode_help">Selects how DTMF tones 0–9, #, *, and A-D are sent.</string>
     <string name="dtmf_inband">In-band RTP Events</string>