Overall, you will need to look at your Network Performance and check whether you have a high degree of congestion. If you have several BHCA periods you may need to look at Load Control features, e.g. Call Gapping, SCCP Congestion Control, etc..
For Telephony if the signalling cannot assign resources then the initiated call will be rejected.
Check the feedback from your OSS / IT systems.
VBR/ Wallis Dudhnath