Old Fashioned Caller ID for Testing

I installed a Elastix phone server for a client about 2 months ago and it seemed to be working fine. Last week they had an issue with the phone company and the roll over for their 4 POTS lines coming in. I got the issue resolved with the phone company, but over the last week they say business has declined and again are thinking they are missing calls because of the phone system.

My Question is, will it interfere with the phone lines if I install 4 old fashioned caller IDs on each of the phone lines between the demarcation point and the Elastix server? Then the client can compare calls on the caller IDs against the phone logs, until they are confident in the system.

If anyone one has any other ideas I would love to hear them.

Thank you in advanced.