diff --git a/q931.py b/q931.py
index f4ca670..99c3f6a 100644
--- a/q931.py
+++ b/q931.py
@@ -117,7 +117,7 @@ TonNpiByte = BitStruct('ext'/Bit,
                        'type_of_number'/TypeOfNumber,
                        'numbering_plan'/NumberingPlan)
 CalledPartyNum = Struct('hdr'/TonNpiByte,
-                        'digits'/GreedyBytes)
+                        'digits'/GreedyString('ascii'))
 
 # Figure 4-15 / Q.931
 OddEvenInd = Enum(Bit, even=0, odd=1)
@@ -138,7 +138,7 @@ CallingPartyNum3a = BitStruct('ext'/Bit,
 
 CallingPartyNum = Struct('hdr'/TonNpiByte,
                          'octet_3a'/If(this.hdr.ext == 0, CallingPartyNum3a),
-                         'digits'/GreedyBytes)
+                         'digits'/GreedyString('ascii'))
 
 # Q.931 Section 4.6.7; Figure 4-43 + Table 4-32: Redirecting number
 ReasonForRedir = Enum(BitsInteger(4), unknown=0, call_forwrding_busy_or_called_dte_busy=1,
@@ -150,7 +150,7 @@ RedirectingNum3b = BitStruct('ext'/Const(1, Bit),
 RedirectingNum = Struct('hdr'/TonNpiByte,
                         'octet_3a'/If(this.hdr.ext == 0, CallingPartyNum3a),
                         'octet_3b'/If(this.hdr.ext == 0 and this.octet_3a.ext == 0, RedirectingNum3b),
-                        'digits'/GreedyBytes)
+                        'digits'/GreedyString('ascii'))
 
 
 InterfaceId = BitStruct('ext'/Bit, 'interface_id'/BitsInteger(7)) # TODO: extension