Short ring on Queue call

I am experiencing the following behaviour when there are for example 3 agents logged into a queue. If you just let the phones ring and do not answer them it will ring Agent 1 for 15 seconds then it will ring Agent 2 for half a second (just half a ring) and report on the console nobody picked up in 15000ms, then it will ring Agent 3 for 15 secs. Then every other ring to an agent is the short ring.

Any ideas why this is happening?

timeout = 15
retry = 5

I tried with timeoutrestart=yes but it didn’t help.