Macro-exten-vm Warnings

We receive multiple warning messages from calls coming in through one trunk on our system:

chan_sip.c: This function can only be used on SIP channels.

These are all being generated when executing macro-exten-vm. The trunk the calls are coming in on is an IAX2 trunk from another asterisk system, so the messages make sense. They also do not seem to cause any call problems that I can detect. However, they are annoying; and I am not 100% certain that they are not causing problems.

Has anyone seen this behavior before? Can anyone suggest something I can look at to bypass this problem?

Thanks for any suggestions.