Here is a link to the config guide.
http://www.cisco.com/en/US/partner/docs/security/asa/asa82/configuration/guide/unified_comm_phoneproxy.html
Based on my understanding, because the ASA requires that URLs leverage HTTPs you need to have a web proxy available to handle the URL requests for XML-based apps from the IP phone (e.g. EM, corporate directory, etc.).
You can configure this as follows:
phone-proxy asa_phone_proxy
proxy-server address 128.106.254.9 interface outside
This aspect of the configuration is documented in bug/defect id: CSCsx08256
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Bug Details
Remote Phones can not use services without external proxy server
Symptom:
Phone registered to CUCM via internet using Phone Proxy can not use service key or corporate directory
Conditions:
Phone is registered to CUCM via internet using Phone proxy
Workaround:
Install external proxy server.
Configure Proxy Server address in ASA Phone Proxy
Please refer to ASA Phone Proxy Configuration guide for more detail
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HTH.
Regards,
Bill
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