SIP Broker is a free service that allows you to set up an account that will enable you to dial any SIP phone with just your numeric keypad.
This service allows you to bypass more complex SIP phonebooks and speed dials.
You can setup up a SIP Broker number such as 1-555-1234 to map to your existing SIP URI and which allows your callers to simply dial this number.
This number can be used even if you change providers. It stays with you forever so you don't have to change numbers when you change providers.
This service is not a replacement for your VOIP provider but a free add on service.
VOIP providers can also enjoy the benefits of having a 3 digit SIP code assigned to their domain enabling other callers to reach their members with just 3 extra digits.
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This website allows you to test your net connection for VOIP capability. Requires Java.