[ERROR] message

I’ve got a big trouble with a telephone.
Asterisk v.1.4.4
3 analog phones are connected to ATA (grandstream)
1 grandstream gpx2000
1 linksys spa922

one analog phone has the following problems:

  • it has no ring tone when receive a call and lines appear busy!

the message error founds for two days is:

ERROR[XXXXX]: Call to peer ‘XX’ rejected due to usage limit of 2
couldn’t call XX

Someone has discovered the same problem? Any idea?