On analog lines, you have no direct control over the outbound caller ID. Each line has a number and that is what will be sent when you call out on that line.
I am guessing that one or more of your lines have 587 numbers.
The ‘r’ tells the system to use the lines in round-robin order. If you make several calls, each will show a different number. See
If that’s not what you want, you can tell the system to select the first available line, or you can set up a trunk to use a specific line (with possible failover to a different line).
However, if you are having this trouble on a PRI, you should be able to fix it in the DAHDI config.