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For the Subscription URL field, a default value should be displayed next to the field. That value should be (Default: sip:~~rl~C~xxx@sipdomain.tld), where xxx is replaced with your extension, and sipdomain.tld is replaced with your SIP domain. Copy and paste the ~~rl~C~xxx@sipdomain.tld entry in from the default hint in the Admin Web interface into the Subscription URL field.
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