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@AI Evy CIDR UK Twilio. I've added the 3.42.144.0/23 but not the 153.57.128.0/18. What is 153.57.128.0/18 used for? Do I need ot add it ?
a
Thanks for tagging me @Ciaran. I'll look through relevant sources and get back to you shortly.
### Answer Retell's SIP server uses multiple IP blocks for traffic [(1) ]() . The 
153.57.128.0/18
block is listed as one of the IP blocks used for traffic to Retell's SIP server, along with 
18.98.16.120/30
(All regions) , 
3.42.144.0/23
(All regions) , 
143.223.88.0/21
(certain United States traffic) , and 
161.115.160.0/19
(certain United States traffic) . Yes, you should add 
153.57.128.0/18
 to ensure complete connectivity. The documentation lists it as one of the IP blocks for traffic across all regions . While you've already added 
3.42.144.0/23
, adding all the IP blocks ensures your Twilio SIP trunk can properly route traffic to Retell's SIP server without connectivity issues. For UK-based Twilio integration, it's recommended to whitelist all the IP blocks listed in the documentation to avoid any potential routing problems .
​ To learn more, see the sources I considered: [[1] Connect Retell voice agents to custom telephony](https://docs.retellai.com/deploy/custom-telephony) ​
Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.
c
@AI Evy So I should have 3 IP ACL's? 153.57.128.0/18, 3.42.144.0/23 and 18.98.16.120/30?
a
Thanks for the follow-up, @Ciaran. I'll get back to you soon.
### Answer Yes, you should whitelist all the IP blocks that Retell uses for traffic [(1) ]() . According to the documentation, there are five IP blocks total : - 
18.98.16.120/30
(All regions) - 
3.42.144.0/23
(All regions) - 
153.57.128.0/18
(All regions) - 
143.223.88.0/21
(certain United States traffic) - 
161.115.160.0/19
(certain United States traffic) So you should have at minimum the three you mentioned (
18.98.16.120/30
, 
3.42.144.0/23
, and 
153.57.128.0/18
) . However, to ensure complete coverage and avoid any potential connectivity issues, it's recommended to add all five IP blocks to your Twilio IP ACL configuration . These IP blocks are used to whitelist traffic to Retell's SIP server .
​ To learn more, see the sources I considered: [[1] Connect Retell voice agents to custom telephony](https://docs.retellai.com/deploy/custom-telephony) ​
Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.
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