It’s recommended so that you don’t have to give all your contacts a new number.
Can I Keep My Phone Number With VoIP?
Switching to VoIP means changing phone service providers and also phone numbers.

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Can you still use your existing landline PSTN phone number with your new VoIP service?
Will yourVoIP service providerallow you to keep your existing phone number?
However, there are limitations.
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VoIP Number Transfer Restrictions
This feature is not always free.
Some VoIP companies offer number portability for a fee.
Besides the fee, porting a number may impose certain restrictions.
You might be barred from benefiting from certain features offered with the new service.
One way people avoid this restriction is to pay for a second line that carries their ported number.
What Is Number Portability?
Number portability is the ability to use your phone number from one phone service provider with another.
This is possible between phone service provider companies, whether they offer wired or wireless service.
A phone number is always attached to a name and an address.
They will have to use a new number obtained from the new company.