Let us first fix some terminology:

  • VOIP (Voice over Internet Protocol) stands for Internet Telephony.
  • The leading protocol to set up (to initiate, alter and terminate) a VOIP phone call is the Session Initiation Protocol (SIP).
  • A phone number provided by a SIP provider is a DID number, a Direct Inward Dialing number.
  • a private branch exchange (PBX) distributes calls between many inside lines (extensions) to a single outside line (trunk). While a PBX used to be hardware installed at the phone company that provided the outside lines, now it is usually software installed at the client who manages the inside lines.

Forwarding Inward Calls

To forward

  1. your landline number from a (solid but rather expensive) DID number VOIP provider
  2. to your (cell)phone number by another (shadier but rather cheap) affordable VOIP provider
  3. via a virtual phone system, private branch exchange (PBX), such as PBXes.org

follow these instructions:

  1. Obtain a SIP account reachable by a phone number by opening an account at a DID number SIP provider, say Sipgate.

  2. To forward calls from this DID provider Sipgate to another SIP provider (without DID number), say Intervoip, via your virtual PBX,

    1. Open an account at Intervoip.

    2. Open an account at PBXes.org.

    3. Login to PBXes.org and add an incoming (and outgoing) SIP trunk for Sipgate (labeled, say Sipgate) by entering your username (which is your client number), password (for your SIP account which may differ from that of your client account) and the SIP server address, here sipgate.de. Check at your DID provider Sipgate whether the trunk appears in the list of registered devices in the entry Routing/Phones.

    4. Create a global incoming rule that forwards all calls (all day long by enforcing work hours)

      • from your account at the DID provider Sipgate
      • to your account john at the other SIP provider intervoip.com by entering its SIP URI consisting of the account name and SIP server domain, here john@sip.intervoip.com.

  3. To let Intervoip (or any other Betamax reseller) forward the calls from your SIP account to your (cell)phone number:

    1. Download the application running on Microsoft Windows. Since some time, all resellers offer the MobileVoIP app of the Microsoft store. However, still working and available for direct download from other sources are the former branded installers, such as that of VoipBuster.
    2. Once the application installed, started and you are logged on to your SIP account, open the menu entry Options and then in the appearing dialog the list entry Call Settings. In the entry Call forwarding choose always and add your cellphone number (including its international prefix and area code).

Placing Outward Calls

To call others, you can use regular phone calls or a (smartphone) VOIP client directly or via PBXes:

Regular Phone Calls

  1. Call a local access number, of which there are a couple of them in each country, all consultable at the geo page of your Betamax reseller (and with varying numbers among them), and
  2. Dial on your (virtual) phone pad the destination number.

The number is called after the charged tariff has been announced.

Smartphone VOIP Client via UDP

On Android, open source clients are

  • SIPdroid, from the operator of pbxes.org
  • Csipsimple, which is no longer maintained

and various solid commercial ones, such as

  • Acrobits
  • Zoiper which was found by me, many years ago, to be the most stable among those I test drove.

Once installed, set up your account in your VOIP client as indicated on the SIP page.

This setup uses UDP to receive calls. Using TCP to establish a connection instead of UDP, will save battery of your cellphone when the SIP client is idly waiting for incoming calls, as follows:

Smartphone VOIP Client via TCP/IP

More flexible and (cellphone) battery friendly is to:

  1. Add to your virtual PBX at pbxes.org

    • an extension, say 100
    • add an outgoing rule that forwards the calls to your Betamax reseller, say Intervoip by entering your username, password and the SIP server address, say sip.intervoip.com. The rule can, for example, pick the account of that Betamax reseller with the cheapest tariffs to the destination number according to its international prefix (and for national landline calls for chivalry use the DID provider).

  2. In your VOIP client, set up your SIP account at pbxes.org by entering the username john-100 (where the first part before the dash john is the pbxes account name and the last part 100 is the extension number), password and the SIP server address pbxes.com