Sounds to me like either you don't have enough dongles to cover the entire bandwidth between your lowest frequency in the system and your highest frequency in the system.
For instance, if you use one dongle and you have frequencies between 851.100 and 853.000 and then one other frequency that is at 857.000, when SDRTrunk is directed to a capture a voice call on 857.000, it would throw that message because you don't have enough coverage as far as mhz of bandwidth with the number of dongles/devices you are using.
Please indicate what system you are monitoring and what device(s) you are using such as # of RTL dongles, Airspy mini, Airspy R2, an RSP, etc.