modem: Implement voice calls in modem and add voice suite

Change-Id: Ib402effc830db293f27a877658894e454a93a606
diff --git a/src/osmo_gsm_tester/schema.py b/src/osmo_gsm_tester/schema.py
index 6d5f7ad..9b142d3 100644
--- a/src/osmo_gsm_tester/schema.py
+++ b/src/osmo_gsm_tester/schema.py
@@ -82,7 +82,7 @@
     raise ValueError('Unknown Cipher value: %r' % val)
 
 def modem_feature(val):
-    if val in ('sms', 'gprs', 'voicecall', 'ussd'):
+    if val in ('sms', 'gprs', 'voice', 'ussd'):
         return
     raise ValueError('Unknown Modem Feature: %r' % val)