Just had exactly the same experience porting from Vodafone - after years of shoddy service - to 3. I was not under contract but was hit with nearly a month's rental. The relevant T&Cs read as follows:
Suspension / Termination
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5.2
This Contract may be terminated by either party giving to the other at least 28 days written notice or, in the case of porting, on the completion of the porting process.
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5.4
Where the Contract is terminated all Charges will continue to accrue and be payable either until the end of the 28 day notice period (where termination is pursuant to Clause 5.2) or until the date of termination (where termination is pursuant to Clause 5.3). The Customer is advised that in certain circumstances Vodafone may become aware of outstanding Charges after the date of the invoice issued on termination (eg. roaming charges that are subsequently advised to Vodafone by its roaming partners). In these circumstances, Vodafone shall be entitled to raise subsequent invoices and the Customer shall be obliged to discharge all such invoices by the due date specified on the invoices. Notwithstanding the foregoing, if the Contract is terminated under Clauses 5.2 or 5.3 during the Minimum Period, the Customer shall, following Vodafone’s invoice, pay a termination charge calculated as the sum of the monthly tariff charges which would otherwise be payable until the end of the Minimum Period.
I'm intrigued by the reptilian legalese in 5.4, where it says "all Charges will continue to accrue and be payable either until the end of the 28 day notice period (where termination is pursuant to Clause 5.2)". That seems to cover only the first of the two termination types mentioned in clause 5.2.
I wonder if, with sufficient fight, they would admit that termination effected in the case of porting "on the completion of the porting process" represents the end of any billable obligation on the customer.
Will post any success. Don't hold breath though - have generally found Vodafone to be contemptible, underhanded, deliberately accidental in billing and basically unpleasant to deal with.