Calls should try to connect to a volunteer that is available.

This request comes from doing a stress test of yap 4.0.0-beta6 server, with 3 callers and 3 volunteers.

Three callers all called at the same time with three phoneline volunteers. One volunteer got two calls and one did not receive a call. The caller went to voicemail, even though there was an available volunteer.

3 calls to 3 volunteers work if spaced over time, but the volunteers still get incoming calls while they are talking to the first callers. Suggestion: volunteers should not receive additional calls until they are done.