diff --git a/src/ccallsign.cpp b/src/ccallsign.cpp
index 23cdb3e..75f6d5d 100644
--- a/src/ccallsign.cpp
+++ b/src/ccallsign.cpp
@@ -110,22 +110,34 @@ bool CCallsign::IsValid(void) const
         }
     }
     valid &= (iNum < 3);
-    // all remaining char are letter, number or space
+    // all remaining char are letter, number or space (or underscore in extended checks)
     for ( ; i < CALLSIGN_LEN; i++)
     {
+#ifdef EXTENDED_DMRID_CHECKS
+        valid &= IsLetter(m_Callsign[i]) || IsNumber(m_Callsign[i]) || IsSpace(m_Callsign[i]) || IsUnderscore(m_Callsign[i]);
+#else
         valid &= IsLetter(m_Callsign[i]) || IsNumber(m_Callsign[i]) || IsSpace(m_Callsign[i]);
+#endif
     }
     
     // prefix
-    // all chars are number, uppercase or space
+    // all chars are number, uppercase or space (or underscore  in extend checks)
     for ( i = 0; i < CALLSUFFIX_LEN; i++ )
     {
+#ifdef EXTENDED_DMRID_CHECKS
+        valid &= IsLetter(m_Suffix[i]) || IsNumber(m_Suffix[i]) || IsSpace(m_Suffix[i]) || IsUnderscore(m_Suffix[i]);
+#else
         valid &= IsLetter(m_Suffix[i]) || IsNumber(m_Suffix[i]) || IsSpace(m_Suffix[i]);
+#endif
     }
     
     // module
     // is an letter or space
+#ifdef EXTENDED_DMRID_CHECKS
+    valid &= IsLetter(m_Module) || IsSpace(m_Module) || IsNumber(m_Module);
+#else
     valid &= IsLetter(m_Module) || IsSpace(m_Module);
+#endif
     
     // dmrid is not tested, as it can be NULL
     // if station does is not dmr registered
@@ -361,6 +373,13 @@ bool CCallsign::IsLetter(char c) const
     return ((c >= 'A') && (c <= 'Z'));
 }
 
+#ifdef EXTENDED_DMRID_CHECKS
+bool CCallsign::IsUnderscore(char c) const
+{
+    return (c == '_');
+}
+#endif
+
 bool CCallsign::IsSpace(char c) const
 {
     return (c == ' ');
diff --git a/src/ccallsign.h b/src/ccallsign.h
index bd5f118..c0436e3 100644
--- a/src/ccallsign.h
+++ b/src/ccallsign.h
@@ -85,6 +85,9 @@ protected:
     bool IsNumber(char) const;
     bool IsLetter(char) const;
     bool IsSpace(char) const;
+#ifdef EXTENDED_DMRID_CHECKS
+    bool IsUnderscore(char) const;
+#endif
     
 protected:
     // data
diff --git a/src/main.h b/src/main.h
index b07c48e..7fc0a74 100644
--- a/src/main.h
+++ b/src/main.h
@@ -144,6 +144,11 @@
 #define DMRIDDB_PATH                    "/xlxd/dmrid.dat"                   // local file path
 #define DMRIDDB_REFRESH_RATE            180                                 // in minutes
 
+// Extended DMR ID checks ---------------------------------------
+
+//#define EXTENDED_DMRID_CHECKS                                             // Also allow extended "callsigns" like "IPSC_EU2" 
+                                                                            // used for incoming DMR-DL master connections
+
 // Wires-X node database ----------------------------------------
 
 #define YSFNODEDB_USE_RLX_SERVER        1                                   // 1 = use http, 0 = use local file