diff --git a/src/Makefile b/src/Makefile
index 499c19c..86b3c7e 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -42,7 +42,7 @@ main.o: main.c ../include/Phone/phone.h phone.h ../config.h
 	$(CC) -D PREFIX=\"$(PREFIX)\" $(phone_CFLAGS) -c main.c
 
 modem.o: modem.c modem.h ../config.h
-	$(CC) $(phone_CFLAGS) -c modem.c
+	$(CC) -D PREFIX=\"$(PREFIX)\" $(phone_CFLAGS) -c modem.c
 
 phone.o: phone.c ../include/Phone/phone.h modem.h phone.h callbacks.h ../config.h
 	$(CC) -D PREFIX=\"$(PREFIX)\" $(phone_CFLAGS) -c phone.c
diff --git a/src/project.conf b/src/project.conf
index 2404f34..aa5ac0b 100644
--- a/src/project.conf
+++ b/src/project.conf
@@ -31,6 +31,7 @@ cppflags=-D PREFIX=\"$(PREFIX)\"
 
 [modem.c]
 depends=modem.h,../config.h
+cppflags=-D PREFIX=\"$(PREFIX)\"
 
 [phone.c]
 depends=../include/Phone/phone.h,modem.h,phone.h,callbacks.h,../config.h