Use the Cisco Vritual Office solution to establish a VPN tunnel using an ISR881 or ASA5505 back to the data center.
Use Cisco Phone Proxy to establish a TLS tunnel between the phone and an ASA at the data center.
Use the SSL VPN introduced in UCM 8.0(1) and generation three or later TNP phones with an ASA at the data center.
None of the three options are trivial to setup or free.
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