Well, my guess is that during the outage your ATA attempted SIP registration every 30 seconds. When the ISP noticed the flurry of SIP packets (also on alternate ports as you tried different port numbers) they enabled more stringent SIP blocking methods, which may include targeting Voxalot's alternate ports, deep packet inspection, blocking all packets to/from the proxy, making changes to their DNS servers to prevent properly resolving domain names of Voxalot's proxies.
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