Intermittent busy signals

Every 1 in about 50 incoming calls, we will receive a busy signal from our service provider. This doesn’t seem to be isolated to 1 particular caller or extension because if you wait 10-15 seconds and call again, everything works properly. SIP to SIP works just fine, it is when calls get routed to our asterisk server from an outside line such as a cell phone. Has anybody experienced this before? Thanks