You will need to configure your PBX to accept SIP signalling traffic on ports udp/5060, tcp/5060 and tcp/5061 from the following IP addresses:
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185.110.248.8
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185.110.249.8
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185.110.250.8
You will also need to configure your PBX to accept RTP media traffic on ports udp/1024:65534 from the following IP ranges:
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185.110.248.0/26
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185.110.249.0/26
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185.110.250.0/26