Phantom sip channels from SNOM phones?

we’ve got four SNOM phones currently talking to asterisk. i’ve noticed that the phones will sometimes show up on the “sip show channels” report despite not actually being in a call. the output looks like this:

Peer User/ANR Call ID Seq (Tx/Rx) Format (None) 3c26700a138 00101/13803 unknow

has anyone seen this before?