You need a match /permit for 18.27.187.249. I don’t know if that is your proxy or not. If it is, you will only be able to access one endpoint through it, and it isn’t just an outbound proxy. If it is not, this might be part of a range of addresses, and you should find the complete range, from your provider.
18.0.0.0/11 comes back to MIT, and the Via lines indicate there is no proxy involved.
The address does not reverse resolve. That’s not a good sign. Are you sure this call is legitimate? Obviously you should not set match/permit lines for toll fraudsters. On the other hand, a fraudster would likely try a full, probably international, phone number.