Nice snippet from MafooUK over on IRC
Code:
<dialplan>
<extension name="e164_to_GB" continue="true">
<action application="set" data="ignore_display_updates=true" />
<action application="translate" data="${destination_number} e164_to_GB" />
<action application="translate" data="${caller_id_number} e164_to_GB" />
</extension>
</dialplan>
<config>
<configuration name="translate.conf" description="Number Translation Rules">
<profiles>
<profile name="GB_to_e164">
<rule regex="^\+(\d+)$" replace="$1"/>
<rule regex="^00(\d+)$" replace="$1"/>
<rule regex="^0([1-9]\d+)$" replace="44$1"/>
<rule regex="^(\d+)$" replace="$1"/>
</profile>
<profile name="e164_to_GB">
<rule regex="^44(\d{3,6})$" replace="$1"/>
<rule regex="^44(\d+)$" replace="0$1"/>
<rule regex="^(\d+)$" replace="00$1"/>
<rule regex="^\+(\d+)$" replace="00$1"/>
</profile>
</profiles>
</configuration>
</config>