In a submission to Oftel this week, Vodafone complained the proposals to introduce so-called "number portability" next year would "impose an arbitrary deadline" based on technical data which had not yet been given to the phone operators.
Oftel had suggested a temporary move towards portability, which could mean the original phone operator diverting the number to the new company. This would mean the customer had made two calls instead of one. Full mobile number portability is not due until 1999.
Alan Harper, Vodafone's commercial director, said the company could lose out from portability if customers were encouraged to move to other operators.
But he added: "We just don't understand what Oftel's solution is for next year and the timescale is unrealistic."