Cannot hear IVR for incoming call

iam new in asterisk,
i have a question. right now. i link my asterisk to aastra (erricson) MXone. which PRI line is connected to … on the aastra side already setup and running no problem in call in and out through H323 trunk… when someone calls in… aastra will redirect the call to asterisk… and play the asterisk ivr… the problem now is some timesu can hear the voice, sometimes cannot.