[Windows Phone 8 Development and Hacking] thals1992:
Just went through documentation...

Dial string overrides when roaming

Example Code Given:

<?xml version="1.0" encoding="utf-8"?>
<Package xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"
Owner=""
Component=""
SubComponent=""
OwnerType="OEM"
ReleaseType="">
<Components>
<SettingsGroup Path="Phone/PerSimSettings/~__IMSI~/RoamingNumberOverrides">
<Setting Name="DialString/~DialString~" Description="Specify dial strings to override when the user is roaming.">
<RegistrySource Type="REG_SZ" Path="HKEY_LOCAL_MACHINE\Software\Microsoft\Phone\PerSimSettings\~__IMSI~\RoamingNumberOverrides\~DialString~" />
<AccessType Create="Yes" Delete="No" Get="No" Replace="Yes" />
</Setting>
</SettingsGroup>
</Components>
</Package>

Dialer codes for supplementary services

Example Code Given:

<<?xml version="1.0" encoding="utf-8" ?>
<ImageCustomizations xmlns="http://schemas.microsoft.com/embedded/2004/10/ImageUpdate"
Name="SupplementaryServicesDialerCodes"
Description="Use to define new mappings or disable the default mappings for dialer codes."
Owner=""
OwnerType="OEM">

<Static>

<Settings Path="Phone/SupplementaryServiceCodeOverrides">
<-- Select the dialer codes to disable or redefine from the set below
<Setting Name="002" Value="" />
<Setting Name="004" Value="" />
<Setting Name="03" Value="" />
<Setting Name="04" Value="" />
<Setting Name="042" Value="" />
<Setting Name="052" Value="" />
<Setting Name="070" Value="" />
<Setting Name="071" Value="" />
<Setting Name="072" Value="" />
<Setting Name="073" Value="" />
<Setting Name="074" Value="" />
<Setting Name="075" Value="" />
<Setting Name="076" Value="" />
<Setting Name="077" Value="" />
<Setting Name="078" Value="" />
<Setting Name="079" Value="" />
<Setting Name="21" Value="" />
<Setting Name="30" Value="" />
<Setting Name="300" Value="" />
<Setting Name="31" Value="" />
<Setting Name="33" Value="" />
<Setting Name="330" Value="" />
<Setting Name="331" Value="" />
<Setting Name="332" Value="" />
<Setting Name="333" Value="" />
<Setting Name="35" Value="" />
<Setting Name="351" Value="" />
<Setting Name="353" Value="" />
<Setting Name="354" Value="" />
<Setting Name="360" Value="" />
<Setting Name="361" Value="" />
<Setting Name="362" Value="" />
<Setting Name="363" Value="" />
<Setting Name="37" Value="" />
<Setting Name="43" Value="" />
<Setting Name="591" Value="" />
<Setting Name="592" Value="" />
<Setting Name="593" Value="" />
<Setting Name="594" Value="" />
<Setting Name="61" Value="" />
<Setting Name="62" Value="" />
<Setting Name="66" Value="" />
<Setting Name="67" Value="" />
<Setting Name="75" Value="" />
<Setting Name="750" Value="" />
<Setting Name="751" Value="" />
<Setting Name="752" Value="" />
<Setting Name="753" Value="" />
<Setting Name="754" Value="" />
<Setting Name="76" Value="" />
<Setting Name="77" Value="" />
<Setting Name="96" Value="" />
-->
</Settings>

</Static>

</ImageCustomizations>

Dialer codes to launch diagnostic applications

Apparently these are somehow deprecated as it requires an Windows Phone 8.0 Silverlight app.

Predefined dialer codes The following table describes the predefined application dialer codes available. The <Call> in the dialer code sequences represents a press of the Call button on the phone.
Dialer code
Numeric equivalent
Description
##DATA#
##3282#
Displays the network information.
##MFG#
##634#
Calls the manufacturing test tool, if one has been implemented.
##RTN# or ##RTN@
##786# or ##786<Call>
"Return To New"—removes all user-specific information from NVRAM and deactivates the phone.
##MSL#
##675#
Subsidy lock.
##PST# or ##PST@
##778# or ##778<Call>
Product Support Tool—Displays the product support menu to select tasks such as deactivation.