Effective UC Migration Strategies

Depending on customer requirements there are variety of approaches that can be used for migration, selecting the correct migration option depending on the following:

  • Project time lines
  • Acceptable customer impact
  • Customer expectation
  • Cost

During the design phase, the migration strategy should be defined and agreed with the customer depending on the customer requirements, the following section outlines some of these options and the various implications of each:



Parallel build

In this option, a new system is built and provisioned with either new PSTN or existing PSTN with number overlays applied. This allows for gradual migration controlled by the PSTN forwarding and dial plan manipulation using steering codes.

This option has a reduced amount of effort although with increased usage charges and the following implications

  • No on-net internal dialing is available during migration
  • Additional PSTN charges during migration
  • Indirect control of migration by deployment team


  • Simple back out plan if something unexpected happens during the cut over
  •  Enables user familiarization with new system


  • Higher risk factor as all services go live at the same time
  •  Complexity with managing PSTN service migration during tight time window
  •  Large number of test resources required for cutover verification
  • System duplication costs
  • Entire site roll-out costs must be committed at beginning of project
  • Data network must be fully complete before users migrate

Hot Cut

For small migrations or where a large outage window is acceptable to the customer this solution option can be utilised where the old system is effectively turned off and all services migrated in a big bang approach. This option requires the least amount of total effort but has the following implications

  • Careful and detailed coordination is required between all teams
  • Large number of deployment resources are required during the migration activities such as porting, PSTN interconnection and testing
  • Roll back if unforeseen issues are encountered can take an extended period
  • This option does carry an increased risk


  •  Least resource intensive option

  • Fastest migration option


  • High risk with limited capabilities for pre-testing
  • High test overhead at time of cut over
  • Cannot be used in environment where a seamless migration is required

Gradual Migration

This option is suited to large or complex deployments where the other options are not practical. In this method trunking is established between the new cluster(s) and existing cluster(s) detailed dial plan design is required to work out this integration both directory number and E.164, this may require establishing Session Manager (SME) or an SBC.

This option requires the greatest amount of effort but reduces risk and allows the deployment team the greatest amount of control during the deployment with the following implications:

  • Determination of location for PSTN before migration, during migration and post migration. As the PSTN can be retained on the existing cluster till the handset migration is complete or moved to the new cluster in advance and trunked to the existing cluster
  • Significant design with added complexity that results in extensive pre-work and testing is required for successful implementation of this strategy.
  • Depending on who currently manages the existing system, there can be some challenges in establishing trunking between the current system, there are varying levels of features that can be supported


  • Ability to move selective groups of users within large sites
  • Simple roll back if required
  • Once established large number of user can be moved rapidly
  •  Provides option for dual phone deployment
  • Can utilize user driven migration processes


  • Potential for large number of interconnections
  • If using ISDN, CPE is required
  • Detailed dial plan design is required
  • Complex solution that may require multiple vendors
  • Duplicated systems for the duration of the roll out

PCD or Backup and Restore

Prime Collaboration Deployment or backup and restore (DRS) to move and/or upgrade an existing CUCM to the HCS data centre. This does have some advantages as the migration can be quite seamless although extensive investigation of the existing systems is required to understand what other services or 3rd party systems require updates.

With this approach, there is no configuration clean-up process nor standardisation which can cause assurance and lifecycles issues down the track. The overbuild process can be used to provide day to management otherwise some configuration transformation will be required.


Using Over the Top (OTT) can allow for early delivery of services for mobile and remote workers prior to migrating any sites as these users are who are most likely to benefit from mobile and remote collaboration.

This can be done before IP connectivity is established and can drive early adoption.

Choosing a Migration Strategy

The choice of trunking between systems often depends of the capabilities of the 3rd party existing system such as

  • Does the system require upgrade to support SIP and/or does it require additional licensing?
  • Does the existing system have space ISDN interfaces?
  • Does the existing system require licensing for QSIG?
  • Are additional ISDN interfaces required for the new solution?
  • What are the service costs for the establishing this trunking to the existing system?
  • What are the service costs for updating the steering codes for each site?
These questions need to be answered to understand what the actual costs for this migration option and understand what level of feature transparency is required

Site Selection

There are several different approaches to selecting the order of sites to be migrated some site targeting factors are

  • Outdated or remote sites if connectivity is in place
  • High risk sites due of old software, and lack of redundancy and documentation
  • For site(s) with groups or divisions, who will champion the new solution can help to build customer confidence
  • Consider migrating headquarters early in the process to achieve critical mass for your project’s success, due to size, scope of features, density of managers and word-of-mouth.
  • Avoid contact centre sites or sites with hunt groups across multiple sites early in the migration
The advantages and disadvantages need to be contrasted to work out the optimal migration plan.

Yarnlab opinion

We built Wrangler to help facilitate what ever migration strategy you choose although my preference is the gradual migration using Wrangler migration manager
Zane EnglandComment