This is a very general question, and there’s no magic solution.
You need to check your vendor counters (as you’re mentioning U2020 I assume you’re talking about silent calls in Huawei), the counter related to this issue.
You need to check RAT (GSM, WCDMA, LTE?). As you said “nodeB” i assume it is for 3G.
In short, there are a lot of thing to do, but you need to first let your scenario clear. For example, how did you identify silent calls?