DTMF and cell phones

[RandyINSite]

Our customers are reporting a problem with our Plum application.
They complain that when the application calls a cell phone, it will not recognize the DTMF key presses.

Is there something extra required to cause Plum to recognize a key press when the number called is a cell phone?
All our customers are U.S. customers; we are not doing any international calling.

According to our customers, the cell phone customers have NOT been receiving the message that would indicate that Plum detected an answering machine; they are offered the menu choices, but the application does not recognize any key presses.

The relevant portion of the VoiceXML is:

<?xml version="1.0" ?>
2 <!DOCTYPE vxml PUBLIC "-//The Plum Group//DTD VOICEXML 2.1//EN" "/usr/local/plumvp/vxml.dtd">
3 <vxml version="2.0">
4 <property name="inputmodes" value="dtmf"/>
5 <property name="voicename" value="lauren"/>
6 <form id="mainmenu">
7 <var name="callee_type" expr="'?= $calleetype ?'"/>
8 <record cond="callee_type=='answeringmachine'" finalsilence="1000ms">
9 <noinput>
10 <prompt>
11 <voice name="lauren">
12 <prosody rate="-10%">
13 Hello. This is Dr. xxx's office calling to remind
14 (patient name)
15 of your appointment on Monday, <say-as type="date">12:00:00 AM</say-as> at <say-as type="time">12:00:00 AM</say-as>
16 at our (clinic name) clinic.
17 </prosody>
18 </voice>
19 </prompt>
20 <disconnect/>
21 <exit/>
22 </noinput>
23 </record>
24 <field name="menuchoice">
25 <grammar type="application/x-jsgf" mode="dtmf">
26 1|2|3|4|5
27 </grammar>
28 <prompt>
29 <voice name="lauren">
30 <prosody rate="-10%">
31 Hello. This is Dr. xxx's office calling to remind
32 (patient name)
33 of your appointment on Monday, <say-as type="date">12:00:00 AM</say-as> at <say-as type="time">12:00:00 AM</say-as>
34 at our (clinic name) clinic.
35 Please press 1 to confirm this appointment, 2 to Reschedule this appointment, 3 to cancel this appointment, 4 to speak to a clinic staff member and 5 to repeat these options.</prosody>
36 </voice>
37 </prompt>
38 <filled>
39 <if cond="menuchoice==1">
40 <prompt bargein="false">

[support]

Hi,

Have you been able to reproduce this IVR issue reliably?

When we tested entering DTMF digits for IVR outbound calls made to our cell phones, we have found that DTMF input on our cell phones seems to be fine.

Regards,
Plum Support