"Since the merger of Vodafone and 3 in June 2009, we've been working to bring all vendors and systems together under one roof, and in many instances the best way to capture our new benefits of scale is to consolidate to a single vendor for essential services to customers. The consolidation to a single voicemail vendor is part of this business integration program."

The spokesperson said the migration --carried out between 2am and 4am on Sunday, 15 May to minimise interruptions -- went "fairly smoothly" but that some iPhone users on the telco's network had experienced a minor inconvenience.

"We did encounter a change which meant that some iPhone customers needed to reboot their phones -- simply by turning their handset off, then turning it on again -- before they would start to receive new voicemail messages," the spokesperson said.

"We were able to use our database to identify and isolate these customers and we sent them a proactive SMS on Sunday to alert them to the need to reboot their handsets."

The telco also intends to store its customers' old voicemail messages on the previous Unisys system for up to four weeks prior to deleting them. Customers can either have the messages transferred to the new system or can download them as .wav files.