For some strange reason, when SIP users call into voicemail, they cannot interact with it.
Anyone have any ideas?
if you mean taht when they push buttons the VM system does not respond, then you have a DTMF problem. with SIP, there are three ways of sending DTMF: inband (send tones as audio along with the voice, only works with ulaw/alaw codec); rfc2833/rtp (sends the tones attached to the RTP media stream) or SIP INFO (sends the tones as SIP INFO messages along the control channel). If you and your provider do not both have the same method set, then your * box will not respond to DTMF tones, which will make your voicemail app nonfunctional.
hope that helps 
IronHelix is mostly right on this issue…
However, in some cases, you and your provider can have different DTMF tones set and it does work… Asterisk does some kind of translation. This is how I am running it.
This doesn’t sound like your problem, though, since the individuals are internal SIP users and aren’t even jumping onto your provider.
Your problem is likely a DTMF/Codec problem. Some codecs do not allow certain types of DTMF. Inband DTMF only works on g711 codecs, from what I understand. On older builds of Asterisk, dtmf info also had problems with the voicemail. This seems to be corrected now, however.
Your best bet is to use rfc2833 as your dtmf. In your sip file, just set dtmf=rfc2833 and see if this solves your problem.
Tim
That did it.